OCP之黄伟 2

51qsx

Restricting and Sorting Data

Objectives
　　After completing this lesson,you should be able to do the following:

• Limit the rows that are retrieved by a query
  
• Sort the rows that are retrieved by a query
  

Character Strings and Dates
　　列别名应当使用双引号
　　字符串拼接或者日期信息，以及where子句中的数据，需要使用单引号

• Character strings and date values are enclosed by single quotation marks
  
• Character values are case-sensitive,and date values are format-sensitive.
  
• The default date format is DD-MON-RR.
  

SQL> SELECT last_name,job_id,department_id
2  FROM employees
3  WHERE last_name = 'Grant';
LAST_NAME                 JOB_ID     DEPARTMENT_ID
------------------------- ---------- -------------
Grant                     SH_CLERK              50
Grant                     SA_REP
SQL> SELECT last_name,job_id,department_id
2  FROM employees
3  WHERE last_name = 'grant';
no rows selected
View Code
Using the LIKE Condition

• Use the LIKE condition to perform wildcard searches of valid search string values.
  
• Search conditions can contain either literal character or numbers:
  
  
  
  • % denotes zero or many characrters.
    
  • _ denotes one character
    
  
  
  
• You can combine pattern-matching characters
  

SELECT last_name FROM employees WHERE last_name LIKE '_o%';
　　

• You can use the ESCAPE identifier to search for the actual % and _ symbols.　
  

SELECT employee_id,last_name,job_id
FROM employees
WHERE job_id LIKE '%SA\_%' ESCAPE '\';
　　
　　

Using the NULL Conditions
　　null value的比较只能使用IS NULL或者IS NOT NULL而不能使用=,符号进行比较.