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  • 查询的使用方法

    Delete from 表名 where 列名 = ‘’

    删除

    Update 表名 set  列名= ‘’  where 列名 = ‘’

    改动

    Update car ser price=price*0.9 where price >30

    查询

    Select * from 表名  全部列

    Select 列名1,列名2  from  表名     投影

    Select * from 表名 where 条件      筛选

    1. 等值与不等值

      Select * from car where code= ‘c001’

      Select * from var where code != ‘c001’

      Select * from car where code in (‘c001’,’c003’)

    2. 模糊查

      Select * from car where name like ‘宝马%’   %为任意多个任意字符

      Select * from car where name like ‘__5%’第三个字母是5  下划线代表的是一个任意字符

      Select * from car where name like ‘%5%’ 包括5

       

       

       

    3. 排序

       Select * from 表名 where...   Order by 列名(ASC/DESC) 升序或降序 列名(ASC/DESC

       Select *from car oeder by brand asc , price desc...  

      统计查询:

      聚合函数

      Select sum (price<列名>) from car . = 查询所有的价格之和 sum()  求和

      Select count (*)  from car where class="95031" .  = 统计数据的条数

      Select max (code) from car = 求最大值

      Select min (price) from car  = 求最小值

      Select avg (price) from car = 求平均值

      分页查询

      每页显示5条数据,取第n页的数据

      Select * from car limit n-1)*5,5  第一个数字是跳过几条数据,第二个数字是取几条数据。

      去重查询

      Select distinct 列名 from car  去掉重复的关键字

      分组查询

      Select count (*) , brand from car group by brand select brand from car group by brand having count (*)>3

      分组之后根据条件查询使用having 不使用where

       连接查询  对列的扩展

      Select * from info join nation on info.nation = nation code

      Select * from info ,nation

      联合查询 对行的扩展

      Select code,name from info

      union

      Select code,name from nation

      子查询

    1. 无关子查询

      外层查询(里层查询)

      子查询的结果当作父查询的条件

      子查询:Select code from nation where name= ‘汉族’

      父查询:select * from info where nation = ‘(子查询)’

    2. 相关子查询

         查询汽车表中油耗低于该系列平均油耗的所有汽车信息。

      子查询:Select * from car where oil <(该系列平均油耗)

      父查询:Select avg (oil) from car where brand = ‘某个系列’

      Select * from car a where oil <(select avg (oil) from car b where b.brand= ‘a.brand’)

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  • 原文地址:https://www.cnblogs.com/qz1234/p/5539571.html
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