SQL使用实例

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　　一、概述
　　
　　Keep in Mind That...
　　
　　SQL is not case sensitive
　　Semicolon after SQL Statements?
　　
　　Some database systems require a semicolon at the end of each SQL statement.
　　
　　Semicolon is the standard way to separate each SQL statement in database systems that allow more than one SQL statement to be executed in the same call to the server.
　　
　　We are using MS Access and SQL Server 2000 and we do not have to put a semicolon after each SQL statement, but some database programs force you to use it.
　　
　　二、分类
　　
　　SQL DML and DDL
　　
　　SQL can be divided into two parts: The Data Manipulation Language (DML) and the Data Definition Language (DDL).
　　
　　The query and update commands form the DML part of SQL:
　　
　　SELECT - extracts data from a database
　　UPDATE - updates data in a database
　　DELETE - deletes data from a database
　　INSERT INTO - inserts new data into a database
　　The DDL part of SQL permits database tables to be created or deleted. It also defines indexes (keys), specifies links between tables, and imposes constraints between tables. The most important DDL statements in SQL are:
　　
　　CREATE DATABASE - creates a new database
　　ALTER DATABASE - modifies a database
　　CREATE TABLE - creates a new table
　　ALTER TABLE - modifies a table
　　DROP TABLE - deletes a table
　　CREATE INDEX - creates an index (search key)
　　DROP INDEX - deletes an index
　　
　　
　　三、各种SQL语句使用方法举例
　　
　　This chapter will explain the SELECT and the SELECT * statements.
　　The asterisk (*) is a quick way of selecting all columns!
　　
　　
　　
　　The SQL SELECT DISTINCT Statement
　　
　　In a table, some of the columns may contain duplicate values. This is not a problem, however, sometimes you will want to list only the different (distinct) values in a table.
　　
　　The DISTINCT keyword can be used to return only distinct (different) values.
　　SELECT DISTINCT City FROM Persons
　　
　　
　　The WHERE clause is used to filter records.
　　
　　The WHERE Clause 
　　
　　The WHERE clause is used to extract only those records that fulfill a specified criterion.
　　SELECT * FROM Persons
　　WHERE City='Sandnes'
　　Quotes Around Text Fields
　　
　　SQL uses single quotes around text values (most database systems will also accept double quotes).
　　
　　However, numeric values should not be enclosed in quotes.
　　
　　For text values:
　　
　　This is correct:
　　
　　SELECT * FROM Persons WHERE FirstName='Tove'
　　
　　This is wrong:
　　
　　SELECT * FROM Persons WHERE FirstName=Tove
　　For numeric values:
　　
　　This is correct:
　　
　　SELECT * FROM Persons WHERE Year=1965
　　
　　This is wrong:
　　
　　SELECT * FROM Persons WHERE Year='1965'
　　
　　
　　Operators Allowed in the WHERE Clause
　　
　　With the WHERE clause, the following operators can be used:
　　
　　OperatorDescription
　　=Equal
　　<>Not equal
　　>Greater than
　　<Less than
　　>=Greater than or equal
　　<=Less than or equal
　　BETWEENBetween an inclusive range
　　LIKESearch for a pattern
　　INTo specify multiple possible values for a column
　　Note: In some versions of SQL the <> operator may be written as !=
　　
　　
　　Combining AND & OR
　　
　　You can also combine AND and OR (use parenthesis to form complex expressions).
　　
　　Now we want to select only the persons with the last name equal to "Svendson" AND the first name equal to "Tove" OR to "Ola":
　　
　　We use the following SELECT statement:
　　
　　SELECT * FROM Persons WHERE
　　LastName='Svendson'
　　AND (FirstName='Tove' OR FirstName='Ola')
　　
　　
　　The ORDER BY Keyword
　　
　　The ORDER BY keyword is used to sort the result-set by a specified column.
　　
　　The ORDER BY keyword sort the records in ascending order by default.
　　
　　If you want to sort the records in a descending order, you can use the DESC keyword.
　　SELECT * FROM Persons
　　ORDER BY LastName DESC
　　
　　
　　The INSERT INTO Statement
　　
　　The INSERT INTO statement is used to insert a new row in a table.
　　
　　SQL INSERT INTO Syntax
　　
　　It is possible to write the INSERT INTO statement in two forms.
　　
　　The first form doesn't specify the column names where the data will be inserted, only their values:
　　
　　INSERT INTO table_name
　　VALUES (value1, value2, value3,...)
　　The second form specifies both the column names and the values to be inserted:
　　
　　INSERT INTO table_name (column1, column2, column3,...)
　　VALUES (value1, value2, value3,...)
　　
　　
　　
　　
　　The UPDATE Statement
　　
　　The UPDATE statement is used to update existing records in a table.
　　
　　SQL UPDATE Syntax
　　
　　UPDATE table_name
　　SET column1=value, column2=value2,...
　　WHERE some_column=some_value
　　Note: Notice the WHERE clause in the UPDATE syntax. The WHERE clause specifies which record or records that should be updated. If you omit the WHERE clause, all records will be updated!
　　
　　Be careful when updating records. If we had omitted the WHERE clause in the example above, like this:
　　
　　UPDATE Persons
　　SET Address='Nissestien 67', City='Sandnes'
　　
　　
　　
　　The DELETE Statement
　　
　　The DELETE statement is used to delete rows in a table.
　　
　　SQL DELETE Syntax
　　
　　DELETE FROM table_name
　　WHERE some_column=some_value
　　Note: Notice the WHERE clause in the DELETE syntax. The WHERE clause specifies which record or records that should be deleted. If you omit the WHERE clause, all records will be deleted!
　　
　　Delete All Rows
　　
　　It is possible to delete all rows in a table without deleting the table. This means that the table structure, attributes, and indexes will be intact:
　　
　　DELETE FROM table_name
　　
　　or
　　
　　DELETE * FROM table_name
　　Note: Be very careful when deleting records. You cannot undo this statement!
　　
　　
　　
　　The TOP Clause
　　
　　The TOP clause is used to specify the number of records to return.
　　
　　The TOP clause can be very useful on large tables with thousands of records. Returning a large number of records can impact on performance.
　　
　　Note: Not all database systems support the TOP clause.
　　SQL SELECT TOP Equivalent in MySQL and Oracle
　　
　　MySQL Syntax
　　
　　SELECT column_name(s)
　　FROM table_name
　　LIMIT number
　　Example
　　
　　SELECT *
　　FROM Persons
　　LIMIT 5
　　Oracle Syntax
　　
　　SELECT column_name(s)
　　FROM table_name
　　WHERE ROWNUM <= number
　　Example
　　
　　SELECT *
　　FROM Persons
　　WHERE ROWNUM <=5
　　
　　The LIKE Operator
　　
　　The LIKE operator is used to search for a specified pattern in a column.
　　
　　Now we want to select the persons living in a city that starts with "s" from the table above.
　　
　　We use the following SELECT statement:
　　
　　SELECT * FROM Persons
　　WHERE City LIKE 's%'
　　The "%" sign can be used to define wildcards (missing letters in the pattern) both before and after the pattern.
　　
　　Next, we want to select the persons living in a city that contains the pattern "tav" from the "Persons" table.
　　
　　We use the following SELECT statement:
　　
　　SELECT * FROM Persons
　　WHERE City LIKE '%tav%'
　　
　　
　　
　　
　　
　　
　　WildcardDescription
　　%A substitute for zero or more characters
　　_A substitute for exactly one character
　　[charlist]Any single character in charlist
　　[^charlist]
　　or
　　
　　[!charlist]
　　
　　Any single character not in charlist
　　Using the [charlist] Wildcard
　　
　　Now we want to select the persons with a last name that starts with "b" or "s" or "p" from the "Persons" table.
　　
　　We use the following SELECT statement:
　　
　　SELECT * FROM Persons
　　WHERE LastName LIKE '[bsp]%'
　　
　　Next, we want to select the persons with a last name that do not start with "b" or "s" or "p" from the "Persons" table.
　　
　　We use the following SELECT statement:
　　
　　SELECT * FROM Persons
　　WHERE LastName LIKE '[!bsp]%'
　　
　　
　　
　　Using the _ Wildcard
　　
　　Now we want to select the persons with a first name that starts with any character, followed by "la" from the "Persons" table.
　　
　　We use the following SELECT statement:
　　
　　SELECT * FROM Persons
　　WHERE FirstName LIKE '_la'
　　
　　
　　
　　
　　
　　The IN Operator
　　
　　The IN operator allows you to specify multiple values in a WHERE clause.
　　Now we want to select the persons with a last name equal to "Hansen" or "Pettersen" from the table 
　　
　　above.
　　
　　We use the following SELECT statement:
　　
　　SELECT * FROM Persons
　　WHERE LastName IN ('Hansen','Pettersen')
　　
　　The BETWEEN Operator
　　
　　The BETWEEN operator selects a range of data between two values. The values can be numbers, text, or dates.
　　
　　
　　Now we want to select the persons with a last name alphabetically between "Hansen" and "Pettersen" from the table above.
　　
　　We use the following SELECT statement:
　　
　　SELECT * FROM Persons
　　WHERE LastName
　　BETWEEN 'Hansen' AND 'Pettersen'
　　Note: The BETWEEN operator is treated differently in different databases!
　　
　　In some databases, persons with the LastName of "Hansen" or "Pettersen" will not be listed, because the BETWEEN operator only selects fields that are between and excluding the test values.
　　
　　In other databases, persons with the LastName of "Hansen" or "Pettersen" will be listed, because the BETWEEN operator selects fields that are between and including the test values.
　　
　　And in other databases, persons with the LastName of "Hansen" will be listed, but "Pettersen" will not be listed (like the example above), because the BETWEEN operator selects fields between the test values, including the first test value and excluding the last test value.
　　
　　Therefore: Check how your database treats the BETWEEN operator.
　　
　　To display the persons outside the range in the previous example, use NOT BETWEEN:
　　
　　SELECT * FROM Persons
　　WHERE LastName
　　NOT BETWEEN 'Hansen' AND 'Pettersen'
　　
　　
　　
　　SQL Alias
　　
　　You can give a table or a column another name by using an alias. This can be a good thing to do if you have very long or complex table names or column names.
　　
　　An alias name could be anything, but usually it is short.
　　Alias Example
　　
　　Assume we have a table called "Persons" and another table called "Product_Orders". We will give the table aliases of "p" and "po" respectively.
　　
　　Now we want to list all the orders that "Ola Hansen" is responsible for.
　　
　　We use the following SELECT statement:
　　
　　SELECT po.OrderID, p.LastName, p.FirstName
　　FROM Persons AS p,
　　Product_Orders AS po
　　WHERE p.LastName='Hansen' AND p.FirstName='Ola'
　　The same SELECT statement without aliases:
　　
　　SELECT Product_Orders.OrderID, Persons.LastName, Persons.FirstName
　　FROM Persons,
　　Product_Orders
　　WHERE Persons.LastName='Hansen' AND Persons.FirstName='Ola'
　　As you'll see from the two SELECT statements above; aliases can make queries easier to both write and to read.
　　
　　
　　
　　SQL JOIN
　　
　　The JOIN keyword is used in an SQL statement to query data from two or more tables, based on a relationship between certain columns in these tables.
　　
　　Tables in a database are often related to each other with keys.
　　
　　A primary key is a column (or a combination of columns) with a unique value for each row. Each primary key value must be unique within the table. The purpose is to bind data together, across tables, without repeating all of the data in every table.
　　Look at the "Persons" table:
　　
　　P_IdLastNameFirstNameAddressCity
　　1HansenOlaTimoteivn 10Sandnes
　　2SvendsonToveBorgvn 23Sandnes
　　3PettersenKariStorgt 20Stavanger
　　Note that the "P_Id" column is the primary key in the "Persons" table. This means that no two rows can have the same P_Id. The P_Id distinguishes two persons even if they have the same name.
　　
　　Next, we have the "Orders" table:
　　
　　O_IdOrderNoP_Id
　　1778953
　　2446783
　　3224561
　　4245621
　　53476415
　　Note that the "O_Id" column is the primary key in the "Orders" table and that the "P_Id" column refers to the persons in the "Persons" table without using their names.
　　
　　Notice that the relationship between the two tables above is the "P_Id" column.
　　
　　Different SQL JOINs
　　
　　Before we continue with examples, we will list the types of JOIN you can use, and the differences between them.
　　
　　JOIN: Return rows when there is at least one match in both tables
　　LEFT JOIN: Return all rows from the left table, even if there are no matches in the right table
　　RIGHT JOIN: Return all rows from the right table, even if there are no matches in the left table
　　FULL JOIN: Return rows when there is a match in one of the tables
　　
　　
　　SQL INNER JOIN Keyword
　　
　　The INNER JOIN keyword return rows when there is at least one match in both tables.
　　Now we want to list all the persons with any orders.
　　
　　We use the following SELECT statement:
　　
　　SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo
　　FROM Persons
　　INNER JOIN Orders
　　ON Persons.P_Id=Orders.P_Id
　　ORDER BY Persons.LastName
　　The result-set will look like this:
　　
　　LastNameFirstNameOrderNo
　　HansenOla22456
　　HansenOla24562
　　PettersenKari77895
　　PettersenKari44678
　　The INNER JOIN keyword return rows when there is at least one match in both tables. If there are rows in "Persons" that do not have matches in "Orders", those rows will NOT be listed.
　　
　　
　　SQL LEFT JOIN Keyword
　　
　　The LEFT JOIN keyword returns all rows from the left table (table_name1), even if there are no matches in the right table (table_name2).
　　PS: In some databases LEFT JOIN is called LEFT OUTER JOIN.
　　Now we want to list all the persons and their orders - if any, from the tables above.
　　
　　We use the following SELECT statement:
　　
　　SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo
　　FROM Persons
　　LEFT JOIN Orders
　　ON Persons.P_Id=Orders.P_Id
　　ORDER BY Persons.LastName
　　The result-set will look like this:
　　
　　LastNameFirstNameOrderNo
　　HansenOla22456
　　HansenOla24562
　　PettersenKari77895
　　PettersenKari44678
　　SvendsonTove 
　　The LEFT JOIN keyword returns all the rows from the left table (Persons), even if there are no matches in the right table (Orders).
　　
　　
　　SQL RIGHT JOIN Keyword
　　
　　The RIGHT JOIN keyword returns all the rows from the right table (table_name2), even if there are no matches in the left table (table_name1).
　　PS: In some databases RIGHT JOIN is called RIGHT OUTER JOIN.
　　Now we want to list all the orders with containing persons - if any, from the tables above.
　　
　　We use the following SELECT statement:
　　
　　SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo
　　FROM Persons
　　RIGHT JOIN Orders
　　ON Persons.P_Id=Orders.P_Id
　　ORDER BY Persons.LastName
　　The result-set will look like this:
　　
　　LastNameFirstNameOrderNo
　　HansenOla22456
　　HansenOla24562
　　PettersenKari77895
　　PettersenKari44678
　　 34764
　　The RIGHT JOIN keyword returns all the rows from the right table (Orders), even if there are no matches in the left table (Persons).
　　
　　SQL FULL JOIN Keyword
　　
　　The FULL JOIN keyword return rows when there is a match in one of the tables
　　Now we want to list all the persons and their orders, and all the orders with their persons.
　　
　　We use the following SELECT statement:
　　
　　SELECT Persons.LastName, Persons.FirstName, Orders.OrderNo
　　FROM Persons
　　FULL JOIN Orders
　　ON Persons.P_Id=Orders.P_Id
　　ORDER BY Persons.LastName
　　The result-set will look like this:
　　
　　LastNameFirstNameOrderNo
　　HansenOla22456
　　HansenOla24562
　　PettersenKari77895
　　PettersenKari44678
　　SvendsonTove 
　　 34764
　　The FULL JOIN keyword returns all the rows from the left table (Persons), and all the rows from the right table (Orders). If there are rows in "Persons" that do not have matches in "Orders", or if there are rows in "Orders" that do not have matches in "Persons", those rows will be listed as well.
　　
　　
　　The SQL UNION Operator
　　
　　The UNION operator is used to combine the result-set of two or more SELECT statements.
　　
　　Notice that each SELECT statement within the UNION must have the same number of columns. The columns must also have similar data types. Also, the columns in each SELECT statement must be in the same order.
　　
　　SQL UNION Example
　　
　　Look at the following tables:
　　
　　"Employees_Norway":
　　
　　E_IDE_Name
　　01Hansen, Ola
　　02Svendson, Tove
　　03Svendson, Stephen
　　04Pettersen, Kari
　　"Employees_USA":
　　
　　E_IDE_Name
　　01Turner, Sally
　　02Kent, Clark
　　03Svendson, Stephen
　　04Scott, Stephen
　　Now we want to list all the different employees in Norway and USA.
　　
　　We use the following SELECT statement:
　　
　　SELECT E_Name FROM Employees_Norway
　　UNION
　　SELECT E_Name FROM Employees_USA
　　The result-set will look like this:
　　
　　E_Name
　　Hansen, Ola
　　Svendson, Tove
　　Svendson, Stephen
　　Pettersen, Kari
　　Turner, Sally
　　Kent, Clark
　　Scott, Stephen
　　Note: This command cannot be used to list all employees in Norway and USA. In the example above we have two employees with equal names, and only one of them will be listed. The UNION command selects only distinct values.
　　
　　SQL UNION ALL Example
　　
　　Now we want to list all employees in Norway and USA:
　　
　　SELECT E_Name FROM Employees_Norway
　　UNION ALL
　　SELECT E_Name FROM Employees_USA
　　Result
　　
　　E_Name
　　Hansen, Ola
　　Svendson, Tove
　　Svendson, Stephen
　　Pettersen, Kari
　　Turner, Sally
　　Kent, Clark
　　Svendson, Stephen
　　Scott, Stephen
　　
　　
　　
　　
　　The SQL SELECT INTO Statement
　　
　　The SELECT INTO statement selects data from one table and inserts it into a different table.
　　
　　The SELECT INTO statement is most often used to create backup copies of tables.
　　
　　SQL SELECT INTO Example
　　
　　Make a Backup Copy - Now we want to make an exact copy of the data in our "Persons" table.
　　
　　We use the following SQL statement:
　　
　　SELECT *
　　INTO Persons_Backup
　　FROM Persons
　　We can also use the IN clause to copy the table into another database:
　　
　　SELECT *
　　INTO Persons_Backup IN 'Backup.mdb'
　　FROM Persons
　　We can also copy only a few fields into the new table:
　　
　　SELECT LastName,FirstName
　　INTO Persons_Backup
　　FROM Persons
　　
　　SQL SELECT INTO - With a WHERE Clause
　　
　　We can also add a WHERE clause.
　　
　　The following SQL statement creates a "Persons_Backup" table with only the persons who lives in the city "Sandnes":
　　
　　SELECT LastName,Firstname
　　INTO Persons_Backup
　　FROM Persons
　　WHERE City='Sandnes'
　　
　　SQL SELECT INTO - Joined Tables
　　
　　Selecting data from more than one table is also possible.
　　
　　The following example creates a "Persons_Order_Backup" table contains data from the two tables "Persons" and "Orders":
　　
　　SELECT Persons.LastName,Orders.OrderNo
　　INTO Persons_Order_Backup
　　FROM Persons
　　INNER JOIN Orders
　　ON Persons.P_Id=Orders.P_Id
　　
　　
　　The CREATE TABLE Statement
　　
　　The CREATE TABLE statement is used to create a table in a database.
　　CREATE TABLE Example
　　
　　Now we want to create a table called "Persons" that contains five columns: P_Id, LastName, FirstName, Address, and City.
　　
　　We use the following CREATE TABLE statement:
　　
　　CREATE TABLE Persons
　　(
　　P_Id int,
　　LastName varchar(255),
　　FirstName varchar(255),
　　Address varchar(255),
　　City varchar(255)
　　)
　　The P_Id column is of type int and will hold a number. The LastName, FirstName, Address, and City columns are of type varchar with a maximum length of 255 characters.
　　
　　
　　
　　Constraints are used to limit the type of data that can go into a table.
　　
　　Constraints can be specified when a table is created (with the CREATE TABLE statement) or after the table is created (with the ALTER TABLE statement).
　　
　　We will focus on the following constraints:
　　
　　NOT NULL
　　UNIQUE
　　PRIMARY KEY
　　FOREIGN KEY
　　CHECK
　　DEFAULT
　　
　　
　　By default, a table column can hold NULL values.
　　
　　SQL NOT NULL Constraint
　　
　　The NOT NULL constraint enforces a column to NOT accept NULL values.
　　
　　The NOT NULL constraint enforces a field to always contain a value. This means that you cannot insert a new record, or update a record without adding a value to this field.
　　
　　The following SQL enforces the "P_Id" column and the "LastName" column to not accept NULL values:
　　
　　CREATE TABLE Persons
　　(
　　P_Id int NOT NULL,
　　LastName varchar(255) NOT NULL,
　　FirstName varchar(255),
　　Address varchar(255),
　　City varchar(255)
　　)
　　
　　
　　
　　
　　SQL UNIQUE Constraint
　　
　　The UNIQUE constraint uniquely identifies each record in a database table.
　　
　　The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.
　　
　　A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.
　　
　　Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.
　　
　　SQL UNIQUE Constraint on CREATE TABLE
　　
　　The following SQL creates a UNIQUE constraint on the "P_Id" column when the "Persons" table is created:
　　
　　MySQL:
　　
　　CREATE TABLE Persons
　　(
　　P_Id int NOT NULL,
　　LastName varchar(255) NOT NULL,
　　FirstName varchar(255),
　　Address varchar(255),
　　City varchar(255),
　　UNIQUE (P_Id)
　　)
　　SQL Server / Oracle / MS Access:
　　
　　CREATE TABLE Persons
　　(
　　P_Id int NOT NULL UNIQUE,
　　LastName varchar(255) NOT NULL,
　　FirstName varchar(255),
　　Address varchar(255),
　　City varchar(255)
　　)
　　To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use the following SQL syntax:
　　
　　MySQL / SQL Server / Oracle / MS Access:
　　
　　CREATE TABLE Persons
　　(
　　P_Id int NOT NULL,
　　LastName varchar(255) NOT NULL,
　　FirstName varchar(255),
　　Address varchar(255),
　　City varchar(255),
　　CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
　　)
　　
　　SQL UNIQUE Constraint on ALTER TABLE
　　
　　To create a UNIQUE constraint on the "P_Id" column when the table is already created, use the following SQL:
　　
　　MySQL / SQL Server / Oracle / MS Access:
　　
　　ALTER TABLE Persons
　　ADD UNIQUE (P_Id)
　　To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use the following SQL syntax:
　　
　　MySQL / SQL Server / Oracle / MS Access:
　　
　　ALTER TABLE Persons
　　ADD CONSTRAINT uc_PersonID UNIQUE (P_Id,LastName)
　　
　　To DROP a UNIQUE Constraint
　　
　　To drop a UNIQUE constraint, use the following SQL:
　　
　　MySQL:
　　
　　ALTER TABLE Persons
　　DROP INDEX uc_PersonID
　　SQL Server / Oracle / MS Access:
　　
　　ALTER TABLE Persons
　　DROP CONSTRAINT uc_PersonID
　　
　　
　　SQL PRIMARY KEY Constraint
　　
　　The PRIMARY KEY constraint uniquely identifies each record in a database table.
　　
　　Primary keys must contain unique values.
　　
　　A primary key column cannot contain NULL values.
　　
　　Each table should have a primary key, and each table can have only ONE primary key.
　　
　　SQL PRIMARY KEY Constraint on CREATE TABLE
　　
　　The following SQL creates a PRIMARY KEY on the "P_Id" column when the "Persons" table is created:
　　
　　MySQL:
　　
　　CREATE TABLE Persons
　　(
　　P_Id int NOT NULL,
　　LastName varchar(255) NOT NULL,
　　FirstName varchar(255),
　　Address varchar(255),
　　City varchar(255),
　　PRIMARY KEY (P_Id)
　　)
　　SQL Server / Oracle / MS Access:
　　
　　CREATE TABLE Persons
　　(
　　P_Id int NOT NULL PRIMARY KEY,
　　LastName varchar(255) NOT NULL,
　　FirstName varchar(255),
　　Address varchar(255),
　　City varchar(255)
　　)
　　To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:
　　
　　MySQL / SQL Server / Oracle / MS Access:
　　
　　CREATE TABLE Persons
　　(
　　P_Id int NOT NULL,
　　LastName varchar(255) NOT NULL,
　　FirstName varchar(255),
　　Address varchar(255),
　　City varchar(255),
　　CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
　　)
　　Note: In the example above there is only ONE PRIMARY KEY (pk_PersonID). However, the value of the pk_PersonID is made up of two columns (P_Id and LastName).
　　SQL PRIMARY KEY Constraint on ALTER TABLE
　　
　　To create a PRIMARY KEY constraint on the "P_Id" column when the table is already created, use the following SQL:
　　
　　MySQL / SQL Server / Oracle / MS Access:
　　
　　ALTER TABLE Persons
　　ADD PRIMARY KEY (P_Id)
　　To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:
　　
　　MySQL / SQL Server / Oracle / MS Access:
　　
　　ALTER TABLE Persons
　　ADD CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)
　　Note: If you use the ALTER TABLE statement to add a primary key, the primary key column(s) must already have been declared to not contain NULL values (when the table was first created).
　　
　　To DROP a PRIMARY KEY Constraint
　　
　　To drop a PRIMARY KEY constraint, use the following SQL:
　　
　　MySQL:
　　
　　ALTER TABLE Persons
　　DROP PRIMARY KEY
　　SQL Server / Oracle / MS Access:
　　
　　ALTER TABLE Persons
　　DROP CONSTRAINT pk_PersonID
　　
　　
　　
　　
　　
　　
　　SQL FOREIGN KEY Constraint
　　
　　A FOREIGN KEY in one table points to a PRIMARY KEY in another table.
　　
　　Let's illustrate the foreign key with an example. Look at the following two tables:
　　
　　The "Persons" table:
　　
　　P_IdLastNameFirstNameAddressCity
　　1HansenOlaTimoteivn 10Sandnes
　　2SvendsonToveBorgvn 23Sandnes
　　3PettersenKariStorgt 20Stavanger
　　The "Orders" table:
　　
　　O_IdOrderNoP_Id
　　1778953
　　2446783
　　3224562
　　4245621
　　Note that the "P_Id" column in the "Orders" table points to the "P_Id" column in the "Persons" table.
　　
　　The "P_Id" column in the "Persons" table is the PRIMARY KEY in the "Persons" table.
　　
　　The "P_Id" column in the "Orders" table is a FOREIGN KEY in the "Orders" table.
　　
　　The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables.
　　
　　
　　
　　
　　
　　SQL CHECK Constraint
　　
　　The CHECK constraint is used to limit the value range that can be placed in a column.
　　
　　If you define a CHECK constraint on a single column it allows only certain values for this column.
　　
　　If you define a CHECK constraint on a table it can limit the values in certain columns based on values in other columns in the row.
　　
　　SQL CHECK Constraint on CREATE TABLE
　　
　　The following SQL creates a CHECK constraint on the "P_Id" column when the "Persons" table is created. The CHECK constraint specifies that the column "P_Id" must only include integers greater than 0.
　　
　　MySQL:
　　
　　CREATE TABLE Persons
　　(
　　P_Id int NOT NULL,
　　LastName varchar(255) NOT NULL,
　　FirstName varchar(255),
　　Address varchar(255),
　　City varchar(255),
　　CHECK (P_Id>0)
　　)
　　SQL Server / Oracle / MS Access:
　　
　　CREATE TABLE Persons
　　(
　　P_Id int NOT NULL CHECK (P_Id>0),
　　LastName varchar(255) NOT NULL,
　　FirstName varchar(255),
　　Address varchar(255),
　　City varchar(255)
　　)
　　To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, use the following SQL syntax:
　　
　　MySQL / SQL Server / Oracle / MS Access:
　　
　　CREATE TABLE Persons
　　(
　　P_Id int NOT NULL,
　　LastName varchar(255) NOT NULL,
　　FirstName varchar(255),
　　Address varchar(255),
　　City varchar(255),
　　CONSTRAINT chk_Person CHECK (P_Id>0 AND City='Sandnes')
　　)
　　
　　SQL CHECK Constraint on ALTER TABLE
　　
　　To create a CHECK constraint on the "P_Id" column when the table is already created, use the following SQL:
　　
　　MySQL / SQL Server / Oracle / MS Access:
　　
　　ALTER TABLE Persons
　　ADD CHECK (P_Id>0)
　　To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, use the following SQL syntax:
　　
　　MySQL / SQL Server / Oracle / MS Access:
　　
　　ALTER TABLE Persons
　　ADD CONSTRAINT chk_Person CHECK (P_Id>0 AND City='Sandnes')
　　
　　To DROP a CHECK Constraint
　　
　　To drop a CHECK constraint, use the following SQL:
　　
　　SQL Server / Oracle / MS Access:
　　
　　ALTER TABLE Persons
　　DROP CONSTRAINT chk_Person
　　MySQL:
　　
　　ALTER TABLE Persons
　　DROP CHECK chk_Person
　　
　　
　　
　　
　　SQL DEFAULT Constraint
　　
　　The DEFAULT constraint is used to insert a default value into a column.
　　
　　The default value will be added to all new records, if no other value is specified.
　　
　　SQL DEFAULT Constraint on CREATE TABLE
　　
　　The following SQL creates a DEFAULT constraint on the "City" column when the "Persons" table is created:
　　
　　My SQL / SQL Server / Oracle / MS Access:
　　
　　CREATE TABLE Persons
　　(
　　P_Id int NOT NULL,
　　LastName varchar(255) NOT NULL,
　　FirstName varchar(255),
　　Address varchar(255),
　　City varchar(255) DEFAULT 'Sandnes'
　　)
　　The DEFAULT constraint can also be used to insert system values, by using functions like GETDATE():
　　
　　CREATE TABLE Orders
　　(
　　O_Id int NOT NULL,
　　OrderNo int NOT NULL,
　　P_Id int,
　　OrderDate date DEFAULT GETDATE()
　　)
　　
　　SQL DEFAULT Constraint on ALTER TABLE
　　
　　To create a DEFAULT constraint on the "City" column when the table is already created, use the following SQL:
　　
　　MySQL:
　　
　　ALTER TABLE Persons
　　ALTER City SET DEFAULT 'SANDNES'
　　SQL Server / MS Access:
　　
　　ALTER TABLE Persons
　　ALTER COLUMN City SET DEFAULT 'SANDNES'
　　Oracle:
　　
　　ALTER TABLE Persons
　　MODIFY City DEFAULT 'SANDNES'
　　
　　To DROP a DEFAULT Constraint
　　
　　To drop a DEFAULT constraint, use the following SQL:
　　
　　MySQL:
　　
　　ALTER TABLE Persons
　　ALTER City DROP DEFAULT
　　SQL Server / Oracle / MS Access:
　　
　　ALTER TABLE Persons
　　ALTER COLUMN City DROP DEFAULT
　　
　　
　　
　　The CREATE INDEX statement is used to create indexes in tables.
　　
　　Indexes allow the database application to find data fast; without reading the whole table.
　　
　　Indexes
　　
　　An index can be created in a table to find data more quickly and efficiently.
　　
　　The users cannot see the indexes, they are just used to speed up searches/queries.
　　
　　Note: Updating a table with indexes takes more time than updating a table without (because the indexes also need an update). So you should only create indexes on columns (and tables) that will be frequently searched against.
　　
　　SQL CREATE INDEX Syntax
　　
　　Creates an index on a table. Duplicate values are allowed:
　　
　　CREATE INDEX index_name
　　ON table_name (column_name)
　　SQL CREATE UNIQUE INDEX Syntax
　　
　　Creates a unique index on a table. Duplicate values are not allowed:
　　
　　CREATE UNIQUE INDEX index_name
　　ON table_name (column_name)
　　Note: The syntax for creating indexes varies amongst different databases. Therefore: Check the syntax for creating indexes in your database.
　　
　　CREATE INDEX Example
　　
　　The SQL statement below creates an index named "PIndex" on the "LastName" column in the "Persons" table:
　　
　　CREATE INDEX PIndex
　　ON Persons (LastName)
　　If you want to create an index on a combination of columns, you can list the column names within the parentheses, separated by commas:
　　
　　CREATE INDEX PIndex
　　ON Persons (LastName, FirstName)
　　
　　
　　
　　
　　
　　
　　
　　
　　
　　
　　
　　Indexes, tables, and databases can easily be deleted/removed with the DROP statement.
　　
　　The DROP INDEX Statement
　　
　　The DROP INDEX statement is used to delete an index in a table.
　　
　　DROP INDEX Syntax for MS Access:
　　
　　DROP INDEX index_name ON table_name
　　DROP INDEX Syntax for MS SQL Server:
　　
　　DROP INDEX table_name.index_name
　　DROP INDEX Syntax for DB2/Oracle:
　　
　　DROP INDEX index_name
　　DROP INDEX Syntax for MySQL:
　　
　　ALTER TABLE table_name DROP INDEX index_name
　　
　　The DROP TABLE Statement
　　
　　The DROP TABLE statement is used to delete a table.
　　
　　DROP TABLE table_name
　　
　　The DROP DATABASE Statement
　　
　　The DROP DATABASE statement is used to delete a database.
　　
　　DROP DATABASE database_name
　　
　　The TRUNCATE TABLE Statement
　　
　　What if we only want to delete the data inside the table, and not the table itself?
　　
　　Then, use the TRUNCATE TABLE statement:
　　
　　TRUNCATE TABLE table_name
　　
　　
　　
　　
　　
　　
　　
　　
　　
　　The ALTER TABLE Statement
　　
　　The ALTER TABLE statement is used to add, delete, or modify columns in an existing table.
　　
　　SQL ALTER TABLE Syntax
　　
　　To add a column in a table, use the following syntax:
　　
　　ALTER TABLE table_name
　　ADD column_name datatype
　　To delete a column in a table, use the following syntax (notice that some database systems don't allow deleting a column):
　　
　　ALTER TABLE table_name
　　DROP COLUMN column_name
　　To change the data type of a column in a table, use the following syntax:
　　
　　My SQL / SQL Server / MS Access:
　　
　　ALTER TABLE table_name
　　ALTER COLUMN column_name datatype
　　Oracle:
　　
　　ALTER TABLE table_name
　　MODIFY column_name datatype
　　
　　SQL ALTER TABLE Example
　　
　　Look at the "Persons" table:
　　
　　P_IdLastNameFirstNameAddressCity
　　1HansenOlaTimoteivn 10Sandnes
　　2SvendsonToveBorgvn 23Sandnes
　　3PettersenKariStorgt 20Stavanger
　　Now we want to add a column named "DateOfBirth" in the "Persons" table.
　　
　　We use the following SQL statement:
　　
　　ALTER TABLE Persons
　　ADD DateOfBirth date
　　Notice that the new column, "DateOfBirth", is of type date and is going to hold a date. The data type specifies what type of data the column can hold. For a complete reference of all the data types available in MS Access, MySQL, and SQL Server, go to our complete Data Types reference.
　　
　　The "Persons" table will now like this:
　　
　　P_IdLastNameFirstNameAddressCityDateOfBirth
　　1HansenOlaTimoteivn 10Sandnes 
　　2SvendsonToveBorgvn 23Sandnes 
　　3PettersenKariStorgt 20Stavanger 
　　
　　Change Data Type Example
　　
　　Now we want to change the data type of the column named "DateOfBirth" in the "Persons" table.
　　
　　We use the following SQL statement:
　　
　　ALTER TABLE Persons
　　ALTER COLUMN DateOfBirth year
　　Notice that the "DateOfBirth" column is now of type year and is going to hold a year in a two-digit or four-digit format.
　　
　　DROP COLUMN Example
　　
　　Next, we want to delete the column named "DateOfBirth" in the "Persons" table.
　　
　　We use the following SQL statement:
　　
　　ALTER TABLE Persons
　　DROP COLUMN DateOfBirth
　　The "Persons" table will now like this:
　　
　　P_IdLastNameFirstNameAddressCity
　　1HansenOlaTimoteivn 10Sandnes
　　2SvendsonToveBorgvn 23Sandnes
　　3PettersenKariStorgt 20Stavanger
　　
　　The FOREIGN KEY constraint also prevents that invalid data form being inserted into the foreign key column, because it has to be one of the values contained in the table it points to.
　　
　　SQL FOREIGN KEY Constraint on CREATE TABLE
　　
　　The following SQL creates a FOREIGN KEY on the "P_Id" column when the "Orders" table is created:
　　
　　MySQL:
　　
　　CREATE TABLE Orders
　　(
　　O_Id int NOT NULL,
　　OrderNo int NOT NULL,
　　P_Id int,
　　PRIMARY KEY (O_Id),
　　FOREIGN KEY (P_Id) REFERENCES Persons(P_Id)
　　)
　　SQL Server / Oracle / MS Access:
　　
　　CREATE TABLE Orders
　　(
　　O_Id int NOT NULL PRIMARY KEY,
　　OrderNo int NOT NULL,
　　P_Id int FOREIGN KEY REFERENCES Persons(P_Id)
　　)
　　To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:
　　
　　MySQL / SQL Server / Oracle / MS Access:
　　
　　CREATE TABLE Orders
　　(
　　O_Id int NOT NULL,
　　OrderNo int NOT NULL,
　　P_Id int,
　　PRIMARY KEY (O_Id),
　　CONSTRAINT fk_PerOrders FOREIGN KEY (P_Id)
　　REFERENCES Persons(P_Id)
　　)
　　
　　SQL FOREIGN KEY Constraint on ALTER TABLE
　　
　　To create a FOREIGN KEY constraint on the "P_Id" column when the "Orders" table is already created, use the following SQL:
　　
　　MySQL / SQL Server / Oracle / MS Access:
　　
　　ALTER TABLE Orders
　　ADD FOREIGN KEY (P_Id)
　　REFERENCES Persons(P_Id)
　　To allow naming of a FOREIGN KEY constraint, and for defining a FOREIGN KEY constraint on multiple columns, use the following SQL syntax:
　　
　　MySQL / SQL Server / Oracle / MS Access:
　　
　　ALTER TABLE Orders
　　ADD CONSTRAINT fk_PerOrders
　　FOREIGN KEY (P_Id)
　　REFERENCES Persons(P_Id)
　　
　　To DROP a FOREIGN KEY Constraint
　　
　　To drop a FOREIGN KEY constraint, use the following SQL:
　　
　　MySQL:
　　
　　ALTER TABLE Orders
　　DROP FOREIGN KEY fk_PerOrders
　　SQL Server / Oracle / MS Access:
　　
　　ALTER TABLE Orders
　　DROP CONSTRAINT fk_PerOrders
　　
　　
　　
　　
　　Auto-increment allows a unique number to be generated when a new record is inserted into a table.
　　
　　AUTO INCREMENT a Field
　　
　　Very often we would like the value of the primary key field to be created automatically every time a new record is inserted.
　　
　　We would like to create an auto-increment field in a table.
　　
　　Syntax for MySQL
　　
　　The following SQL statement defines the "P_Id" column to be an auto-increment primary key field in the "Persons" table:
　　
　　CREATE TABLE Persons
　　(
　　P_Id int NOT NULL AUTO_INCREMENT,
　　LastName varchar(255) NOT NULL,
　　FirstName varchar(255),
　　Address varchar(255),
　　City varchar(255),
　　PRIMARY KEY (P_Id)
　　)
　　MySQL uses the AUTO_INCREMENT keyword to perform an auto-increment feature.
　　
　　By default, the starting value for AUTO_INCREMENT is 1, and it will increment by 1 for each new record.
　　
　　To let the AUTO_INCREMENT sequence start with another value, use the following SQL statement:
　　
　　ALTER TABLE Persons AUTO_INCREMENT=100
　　To insert a new record into the "Persons" table, we will not have to specify a value for the "P_Id" column (a unique value will be added automatically):
　　
　　INSERT INTO Persons (FirstName,LastName)
　　VALUES ('Lars','Monsen')
　　The SQL statement above would insert a new record into the "Persons" table. The "P_Id" column would be assigned a unique value. The "FirstName" column would be set to "Lars" and the "LastName" column would be set to "Monsen".
　　
　　Syntax for SQL Server
　　
　　The following SQL statement defines the "P_Id" column to be an auto-increment primary key field in the "Persons" table:
　　
　　CREATE TABLE Persons
　　(
　　P_Id int PRIMARY KEY IDENTITY,
　　LastName varchar(255) NOT NULL,
　　FirstName varchar(255),
　　Address varchar(255),
　　City varchar(255)
　　)
　　The MS SQL Server uses the IDENTITY keyword to perform an auto-increment feature.
　　
　　By default, the starting value for IDENTITY is 1, and it will increment by 1 for each new record.
　　
　　To specify that the "P_Id" column should start at value 10 and increment by 5, change the identity to IDENTITY(10,5).
　　
　　To insert a new record into the "Persons" table, we will not have to specify a value for the "P_Id" column (a unique value will be added automatically):
　　
　　INSERT INTO Persons (FirstName,LastName)
　　VALUES ('Lars','Monsen')
　　The SQL statement above would insert a new record into the "Persons" table. The "P_Id" column would be assigned a unique value. The "FirstName" column would be set to "Lars" and the "LastName" column would be set to "Monsen".
　　
　　Syntax for Access
　　
　　The following SQL statement defines the "P_Id" column to be an auto-increment primary key field in the "Persons" table:
　　
　　CREATE TABLE Persons
　　(
　　P_Id PRIMARY KEY AUTOINCREMENT,
　　LastName varchar(255) NOT NULL,
　　FirstName varchar(255),
　　Address varchar(255),
　　City varchar(255)
　　)
　　The MS Access uses the AUTOINCREMENT keyword to perform an auto-increment feature.
　　
　　By default, the starting value for AUTOINCREMENT is 1, and it will increment by 1 for each new record.
　　
　　To specify that the "P_Id" column should start at value 10 and increment by 5, change the autoincrement to AUTOINCREMENT(10,5).
　　
　　To insert a new record into the "Persons" table, we will not have to specify a value for the "P_Id" column (a unique value will be added automatically):
　　
　　INSERT INTO Persons (FirstName,LastName)
　　VALUES ('Lars','Monsen')
　　The SQL statement above would insert a new record into the "Persons" table. The "P_Id" column would be assigned a unique value. The "FirstName" column would be set to "Lars" and the "LastName" column would be set to "Monsen".
　　
　　Syntax for Oracle
　　
　　In Oracle the code is a little bit more tricky.
　　
　　You will have to create an auto-increment field with the sequence object (this object generates a number sequence).
　　
　　Use the following CREATE SEQUENCE syntax:
　　
　　CREATE SEQUENCE seq_person
　　MINVALUE 1
　　START WITH 1
　　INCREMENT BY 1
　　CACHE 10
　　The code above creates a sequence object called seq_person, that starts with 1 and will increment by 1. It will also cache up to 10 values for performance. The cache option specifies how many sequence values will be stored in memory for faster access.
　　
　　To insert a new record into the "Persons" table, we will have to use the nextval function (this function retrieves the next value from seq_person sequence):
　　
　　INSERT INTO Persons (P_Id,FirstName,LastName)
　　VALUES (seq_person.nextval,'Lars','Monsen')
　　The SQL statement above would insert a new record into the "Persons" table. The "P_Id" column would be assigned the next number from the seq_person sequence. The "FirstName" column would be set to "Lars" and the "LastName" column would be set to "Monsen"
　　
　　
　　
　　
　　A view is a virtual table.
　　
　　This chapter shows how to create, update, and delete a view.
　　
　　SQL CREATE VIEW Statement
　　
　　In SQL, a view is a virtual table based on the result-set of an SQL statement.
　　
　　A view contains rows and columns, just like a real table. The fields in a view are fields from one or more real tables in the database.
　　
　　You can add SQL functions, WHERE, and JOIN statements to a view and present the data as if the data were coming from one single table.
　　
　　SQL CREATE VIEW Syntax
　　
　　CREATE VIEW view_name AS
　　SELECT column_name(s)
　　FROM table_name
　　WHERE condition
　　Note: A view always shows up-to-date data! The database engine recreates the data, using the view's SQL statement, every time a user queries a view.
　　
　　SQL CREATE VIEW Examples
　　
　　If you have the Northwind database you can see that it has several views installed by default.
　　
　　The view "Current Product List" lists all active products (products that are not discontinued) from the "Products" table. The view is created with the following SQL:
　　
　　CREATE VIEW [Current Product List] AS
　　SELECT ProductID,ProductName
　　FROM Products
　　WHERE Discontinued=No
　　We can query the view above as follows:
　　
　　SELECT * FROM [Current Product List]
　　Another view in the Northwind sample database selects every product in the "Products" table with a unit price higher than the average unit price:
　　
　　CREATE VIEW [Products Above Average Price] AS
　　SELECT ProductName,UnitPrice
　　FROM Products
　　WHERE UnitPrice>(SELECT AVG(UnitPrice) FROM Products)
　　We can query the view above as follows:
　　
　　SELECT * FROM [Products Above Average Price]
　　Another view in the Northwind database calculates the total sale for each category in 1997. Note that this view selects its data from another view called "Product Sales for 1997":
　　
　　CREATE VIEW [Category Sales For 1997] AS
　　SELECT DISTINCT CategoryName,Sum(ProductSales) AS CategorySales
　　FROM [Product Sales for 1997]
　　GROUP BY CategoryName
　　We can query the view above as follows:
　　
　　SELECT * FROM [Category Sales For 1997]
　　We can also add a condition to the query. Now we want to see the total sale only for the category "Beverages":
　　
　　SELECT * FROM [Category Sales For 1997]
　　WHERE CategoryName='Beverages'
　　
　　SQL Updating a View
　　
　　You can update a view by using the following syntax:
　　
　　SQL CREATE OR REPLACE VIEW Syntax
　　
　　CREATE OR REPLACE VIEW view_name AS
　　SELECT column_name(s)
　　FROM table_name
　　WHERE condition
　　Now we want to add the "Category" column to the "Current Product List" view. We will update the view with the following SQL:
　　
　　CREATE VIEW [Current Product List] AS
　　SELECT ProductID,ProductName,Category
　　FROM Products
　　WHERE Discontinued=No
　　
　　SQL Dropping a View
　　
　　You can delete a view with the DROP VIEW command.
　　
　　SQL DROP VIEW Syntax
　　
　　DROP VIEW view_name
　　
　　
　　
　　
　　SQL Dates
　　
　　The most difficult part when working with dates is to be sure that the format of the date you are trying to insert, matches the format of the date column in the database.
　　
　　As long as your data contains only the date portion, your queries will work as expected. However, if a time portion is involved, it gets complicated.
　　
　　Before talking about the complications of querying for dates, we will look at the most important built-in functions for working with dates.
　　
　　MySQL Date Functions
　　
　　The following table lists the most important built-in date functions in MySQL:
　　
　　FunctionDescription
　　NOW()Returns the current date and time
　　CURDATE()Returns the current date
　　CURTIME()Returns the current time
　　DATE()Extracts the date part of a date or date/time expression
　　EXTRACT()Returns a single part of a date/time
　　DATE_ADD()Adds a specified time interval to a date
　　DATE_SUB()Subtracts a specified time interval from a date
　　DATEDIFF()Returns the number of days between two dates
　　DATE_FORMAT()Displays date/time data in different formats
　　
　　SQL Server Date Functions
　　
　　The following table lists the most important built-in date functions in SQL Server:
　　
　　FunctionDescription
　　GETDATE()Returns the current date and time
　　DATEPART()Returns a single part of a date/time
　　DATEADD()Adds or subtracts a specified time interval from a date
　　DATEDIFF()Returns the time between two dates
　　CONVERT()Displays date/time data in different formats
　　
　　SQL Date Data Types
　　
　　MySQL comes with the following data types for storing a date or a date/time value in the database:
　　
　　DATE - format YYYY-MM-DD
　　DATETIME - format: YYYY-MM-DD HH:MM:SS
　　TIMESTAMP - format: YYYY-MM-DD HH:MM:SS
　　YEAR - format YYYY or YY
　　SQL Server comes with the following data types for storing a date or a date/time value in the database:
　　
　　DATE - format YYYY-MM-DD
　　DATETIME - format: YYYY-MM-DD HH:MM:SS
　　SMALLDATETIME - format: YYYY-MM-DD HH:MM:SS
　　TIMESTAMP - format: a unique number
　　Note: The date types are chosen for a column when you create a new table in your database!
　　
　　For an overview of all data types available, go to our complete Data Types reference.
　　
　　SQL Working with Dates
　　
　　You can compare two dates easily if there is no time component involved!
　　
　　Assume we have the following "Orders" table:
　　
　　OrderIdProductNameOrderDate
　　1Geitost2008-11-11
　　2Camembert Pierrot2008-11-09
　　3Mozzarella di Giovanni2008-11-11
　　4Mascarpone Fabioli2008-10-29
　　Now we want to select the records with an OrderDate of "2008-11-11" from the table above.
　　
　　We use the following SELECT statement:
　　
　　SELECT * FROM Orders WHERE OrderDate='2008-11-11'
　　The result-set will look like this:
　　
　　OrderIdProductNameOrderDate
　　1Geitost2008-11-11
　　3Mozzarella di Giovanni2008-11-11
　　Now, assume that the "Orders" table looks like this (notice the time component in the "OrderDate" column):
　　
　　OrderIdProductNameOrderDate
　　1Geitost2008-11-11 13:23:44
　　2Camembert Pierrot2008-11-09 15:45:21
　　3Mozzarella di Giovanni2008-11-11 11:12:01
　　4Mascarpone Fabioli2008-10-29 14:56:59
　　If we use the same SELECT statement as above:
　　
　　SELECT * FROM Orders WHERE OrderDate='2008-11-11'
　　we will get no result! This is because the query is looking only for dates with no time portion.
　　
　　Tip: If you want to keep your queries simple and easy to maintain, do not allow time components in your dates!
　　
　　
　　
　　NULL values represent missing unknown data.
　　
　　By default, a table column can hold NULL values.
　　
　　This chapter will explain the IS NULL and IS NOT NULL operators.
　　
　　SQL NULL Values
　　
　　If a column in a table is optional, we can insert a new record or update an existing record without adding a value to this column. This means that the field will be saved with a NULL value.
　　
　　NULL values are treated differently from other values.
　　
　　NULL is used as a placeholder for unknown or inapplicable values.
　　
　　Note: It is not possible to compare NULL and 0; they are not equivalent.
　　
　　SQL Working with NULL Values
　　
　　Look at the following "Persons" table:
　　
　　P_IdLastNameFirstNameAddressCity
　　1HansenOla Sandnes
　　2SvendsonToveBorgvn 23Sandnes
　　3PettersenKari Stavanger
　　Suppose that the "Address" column in the "Persons" table is optional. This means that if we insert a record with no value for the "Address" column, the "Address" column will be saved with a NULL value.
　　
　　How can we test for NULL values?
　　
　　It is not possible to test for NULL values with comparison operators, such as =, <, or <>.
　　
　　We will have to use the IS NULL and IS NOT NULL operators instead.
　　
　　SQL IS NULL
　　
　　How do we select only the records with NULL values in the "Address" column?
　　
　　We will have to use the IS NULL operator:
　　
　　SELECT LastName,FirstName,Address FROM Persons
　　WHERE Address IS NULL
　　The result-set will look like this:
　　
　　LastNameFirstNameAddress
　　HansenOla 
　　PettersenKari 
　　Tip: Always use IS NULL to look for NULL values.
　　
　　SQL IS NOT NULL
　　
　　How do we select only the records with no NULL values in the "Address" column?
　　
　　We will have to use the IS NOT NULL operator:
　　
　　SELECT LastName,FirstName,Address FROM Persons
　　WHERE Address IS NOT NULL
　　The result-set will look like this:
　　
　　LastNameFirstNameAddress
　　SvendsonToveBorgvn 23
　　In the next chapter we will look at the ISNULL(), NVL(), IFNULL() and COALESCE() functions.
　　
　　
　　
　　SQL ISNULL(), NVL(), IFNULL() and COALESCE() Functions
　　
　　Look at the following "Products" table:
　　
　　P_IdProductNameUnitPriceUnitsInStockUnitsOnOrder
　　1Jarlsberg10.451615
　　2Mascarpone32.5623 
　　3Gorgonzola15.67920
　　Suppose that the "UnitsOnOrder" column is optional, and may contain NULL values.
　　
　　We have the following SELECT statement:
　　
　　SELECT ProductName,UnitPrice*(UnitsInStock+UnitsOnOrder)
　　FROM Products
　　In the example above, if any of the "UnitsOnOrder" values are NULL, the result is NULL.
　　
　　Microsoft's ISNULL() function is used to specify how we want to treat NULL values.
　　
　　The NVL(), IFNULL(), and COALESCE() functions can also be used to achieve the same result.
　　
　　In this case we want NULL values to be zero.
　　
　　Below, if "UnitsOnOrder" is NULL it will not harm the calculation, because ISNULL() returns a zero if the value is NULL:
　　
　　SQL Server / MS Access
　　
　　SELECT ProductName,UnitPrice*(UnitsInStock+ISNULL(UnitsOnOrder,0))
　　FROM Products
　　Oracle
　　
　　Oracle does not have an ISNULL() function. However, we can use the NVL() function to achieve the same result:
　　
　　SELECT ProductName,UnitPrice*(UnitsInStock+NVL(UnitsOnOrder,0))
　　FROM Products
　　MySQL
　　
　　MySQL does have an ISNULL() function. However, it works a little bit different from Microsoft's ISNULL() function.
　　
　　In MySQL we can use the IFNULL() function, like this:
　　
　　SELECT ProductName,UnitPrice*(UnitsInStock+IFNULL(UnitsOnOrder,0))
　　FROM Products
　　or we can use the COALESCE() function, like this:
　　
　　SELECT ProductName,UnitPrice*(UnitsInStock+COALESCE(UnitsOnOrder,0))
　　FROM Products