18. MySQL

1. The + + PRIMARY key (primary key) uniquely identifies a row in the table for a property or attribute group + +. = = A table can have only one primary key = =, but may have multiple candidate indexes. = = Primary key can guarantee record unique = = and = = Primary key domain not empty = =, the database management system automatically generates a unique index for the primary key, so the primary key is also a special index.

2. Remember:
   1. The value of the primary key is unique
   2. A data table can have only one primary key
   3. The value of the primary key cannot be null

Increment: Add a piece of data to the table

INSERT INTO `表名`(`列名1`, `列名2`, `列名3`) VALUES (数字1,‘字符串2‘,‘内容3‘)    insert into 表名(字段1，字段2，……) values(值1，值2，……)    insert into scoretab (sname,h5,js) values(‘$sname‘,$h5,$js)

Exp:

INSERT INTO `users`(`uid`, `uname`, `upwd`) VALUES (5,‘小刀‘,001)

By deleting:

DELETE FROM `表名` WHERE (判断条件)1.  Delete from 表名 清空表数据（慎重使用）2.  Delete from 表名 where 字段=值3.  删除编号为3的产品delete from pro where pid = 34.  删除 小米 手机这个产品delete from pro where name = ‘小米’    delete from `scoretab` where sid=$id

Exp:

DELETE FROM `users` WHERE uid = 6(判断条件)

Change:

UPDATE `表名` SET `列名1`=[value-1],`列名2`=[value-2],`列名3`=[value-3] WHERE (条件)1.  Update 表名 set 字段 = 值, 字段 = 值, …… where 条件2.  修改 小米手机价格是 3000update product set price = 3000 where pname=”小米”    update scoretab set sname =‘$sname‘,h5 = $h5,js = $js where sid = $sid

Exp:

UPDATE `users` SET `upwd`= 666 WHERE uid = 8

Check:

SELECT `uid`, `uname`, `upwd` FROM `users` WHERE (条件)1.  Select * from 表名     查询所有的数据2.  Select pname,price from 表名     查询指定的列3.  Select * from 表名 where 条件    按照条件查询4.  查询编号为7的商品select * from 表名 where pid = 7    select * from scoretab where sid = $id    //通配符,查找所有

Exp:

SELECT `uid`, `uname`, `upwd` FROM `users` WHERE uname = ‘小明‘

18. MySQL