python学习之数据库操作
一、数据库基本操作数据库操作:
查看存储引擎: show engines;
查看数据库: show databases; 或者show create database oldboy\G
创建数据库: create database oldboy default
删除数据库: drop database oldboy;
进入数据库:use oldboy;
数据表操作:
创建数据表:
create table tb_emp1 (
-> id int(11),
-> name varchar(25),
-> deptId int(11),
-> salary float
-> )engine=innodb default charset=utf8;
primary key:
create table tb_emp2 (
-> id int(11) primary key,
-> name varchar(25),
-> deptId int(11),
-> salary float
-> )engine=innodb default charset=utf8;
多个primary key:
create table tb_emp3 ( id int(11), name varchar(25), deptId int(11), salary float,
primary key(name, deptId) )engine=innodb default charset=utf8;
外键约束foreign key:
主表:
create table tb_dept1 (
-> id int(11) primary key,
-> name varchar(22) not null,
-> location varchar(50)
-> )engine=innodb default charset=utf8;
从表:
create table tb_emp5 (
-> id int(11) primary key,
-> name varchar(25),
-> deptId int(11),
-> salary float,
-> constraint fk_emp_dept1 foreign key(deptId) references tb_dept1(id)
-> )engine=innodb default charset=utf8;
非空约束not null:
create table tb_emp6 (
-> id int(11) primary key,
-> name varchar(25) not null, #非空
-> deptId int(11),
-> salary float,
-> constraint fk_emp_dept2 foreign key(deptId) references tb_dept1(id)
-> )engine=innodb default charset=utf8;
唯一性约束unique,要求该列唯一，允许为空，但只能有一个值为空:
create table tb_dept2 (
-> id int(11) primary key,
-> name varchar(22) unique,
-> location varchar(50)
-> )engine=innodb default charset=utf8;
默认值default:
create table tb_emp7 (
-> id int(11) primary key,
-> name varchar(25) not null,
-> deptId int(11) default 1111,
-> salary float,
-> constraint fk_emp_dept3 foreign key(deptId) references tb_dept1(id)
-> )engine=innodb default charset=utf8;
自增主键auto_increment:
create table tb_emp8 (
-> id int(11) primary key auto_increment,
-> name varchar(25) not null,
-> deptId int(11),
-> salary float,
-> constraint fk_emp_dept5 foreign key(deptId) references tb_dept1(id)
-> )engine=innodb default charset=utf8;
插入tb_emp8三条数据:
insert into tb_emp8(name,salary) values('Lucy', 1000),('lura', 1200),('Kevin',1500);
id自增
查看表结构: desc tb_emp8; 或者show create table tb_emp8\G
修改数据表alter
修改表名: alter table tb_dept2 rename tb_deptment3;
修改字段类型: alter table tb_dept1 modify name varchar(30);
修改字段名: alter table tb_dept1 change location loc varchar(50);
添加字段: alter table tb_dept1 add managerId int(10);
添加有约束条件的字段: alter table tb_dept1 add column1 varchar(12) not null;
在某个位置添加字段: alter table tb_dept1 add column2 int(11) first;
在某个字段后面添加新字段: alter table tb_dept1 add column3 int(11) after name;
删除字段: alter table tb_dept1 drop column2;
修改表的存储引擎: alter table tb_deptment3 engine=MyISAM;
删除外键约束: alter table tb_emp9 drop foreign key fk_emp_dept;
删除数据表: drop table if exists tb_dept2;
删除外键关联的主表，需要首先取消外键关联，否则删除主表失败
数据类型
整形: TINYINT 1个字节2**8 - 1=255个值
整形: smallint 2个字节
整形: int 4个字节
整形: bigint 8个字节
浮点数
单精度: float 4个字节
双精度: double 8个字节
decimal: 不固定，一般用于财务系统
日期时间类型
year: 1个字节,格式 YYYY
time: 3个字节,格式 HH:MM:SS
date: 3个字节,格式 YYYY-MM-DD
datetime: 8个字节，格式 YYYY-MM-DD HH:MM:SS
timestamp: 4个字节，格式YYYY-MM-DD HH:MM:SS
字符串类型
char(n): 固定长度字符串 #浪费内存，但查询速度快
varchar(n): 非固定长度字符串 #节省内存，但查询速度慢
text: 存放文本
longtext: 存放大数据文本
between .. and .. 关键字使用
select 4 between 4 and 6, 4 between 4 and 6, 12 between 9 and 10;
in, not in 关键字使用
select 2 in(1,3,5,'thks'), 'thks' in(1,3,5,'thks');
like用来匹配字符串
'%': 匹配任何数目的字符
'_': 只能匹配一个字符
select查询数据
create table fruits (
-> f_id char(10) not null,
-> s_id int not null,
-> f_name char(255) not null,
-> f_price decimal(8,2) not null,
-> primary key(f_id)
-> ) engine -> ) engine=innodb =utf8;
插入字段:
insert into fruits(f_id,s_id,f_name,f_price) values
-> ('a1',101,'apple',5.2),
-> ('b1',102,'blackberry',10.2),
-> ('bs1',102,'orange',11.2),
-> ('bs2',105,'melon',8.2),
-> ('t1',102,'banana',10.3),
-> ('t2',102,'grape',5.3),
-> ('o2',103,'coconut',9.2),
-> ('c0',101,'cherry',3.2),
-> ('a2',103,'apricot',2.2),
-> ('l2',104,'lemon',6.4),
-> ('b2',104,'lemon',7.6),
-> ('m1',106,'mango',15.6),
-> ('m2',105,'xbabay',2.6),
-> ('t4',107,'xbababa',3.6),
-> ('m3',105,'xxtt',11.6),
-> ('b5',107,'xxxx',3.6);
单表查询:
查询表: select f_id,f_name from fruits;
查询条件where:
select f_id,f_name from fruits where f_price=10.2; #等号 =
select * from fruits where f_price < 10; #小于
select * from fruits where s_id in(101,102) order by f_name (desc); #in关键字,按f_name排序，desc降序，asc升序
select * from fruits where f_price between 2.00 and 10.20; #between and
select * from fruits where f_name like 'b%'; #like关键字匹配, %匹配任何多个字符
select * from fruits where f_name like '_____y'; #_匹配任意一个字符
select * from fruits where s_id='101' and f_price >=5; #and多条件匹配
select * from fruits where s_id='101' or s_id=102; #or多条件匹配
order by查询结果排序
select * from fruits order by f_name;
select * from fruits order by f_price desc; #desc倒叙排列
group by分组
select s_id,count(*) as Total from fruits group by s_id; #根据s_id分组,s_id相同的数量
select s_id,count(*) as Total from fruits group by s_id having count(f_name) > 1; #having后面加上查询条件
limit限制查询的数量
select * from fruits limit 4; #查询四条
select * from fruits limit 4,3; #索引为4，从第五条开始返回3条
inner join 内连接,返回两表中都有的记录
create table suppliers (
-> s_id int(11) not null auto_increment primary key,
-> s_name char(50) not null,
-> s_city char(50),
-> s_zip char(10),
-> s_call char(50) not null
-> )engine=innodb default charset=utf8;
以下操作是fruits和suppliers关联
select suppliers.s_id, s_name, f_name, f_price from fruits inner join suppliers on
-> fruits.s_id = suppliers.s_id; #on后面是条件查询，
left join 返回包括左表中的所有记录和右表连接字段的所有记录
select s_name,f_price from fruits left join suppliers on fruits.s_id = suppliers.s_id;
子查询:
select s_id, f_name from fruits where s_id=(select s1.s_id from suppliers as s1 where s1.s_city='Tianjin');
union合并查询结果并去重
union all合并查询不去重
select s_id ,f_name,f_price from fruits where f_price < 9.0 union all select s_id,f_name,f_price from fruits where s_id in(101,103);
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