数据库基础查询方法

Posted 2020-10-08 牙齿有个缺缺

mysql表格查询方法：

查询：

1.简单查询

select * from Info --查所有数据
select Code,Name from Info --查指定列的数据
select Code as ‘代号‘,Name as ‘姓名‘ from Info --给列指定别名

2.条件查询

select * from Info where Code=‘p001‘
select * from Info where Sex=‘true‘ and Nation=‘n001‘ --多条件并的关系
select * from Info where Sex=‘true‘ or Nation=‘n001‘ --多条件或的关系

3.范围查询

select * from Car where Price>40 and Price<50
select * from Car where Price between 40 and 50

4.离散查询

select * from Car where Code in (‘c001‘,‘c005‘,‘c010‘,‘c015‘)
select * from Car where Code not in (‘c001‘,‘c005‘,‘c010‘,‘c015‘)

5.模糊查询

select * from Car where Name like ‘%宝马%‘ --查包含宝马的
select * from Car where Name like ‘宝马%‘ --查以宝马开头的
select * from Car where Name like ‘%宝马‘ --查以宝马结尾的
select * from Car where Name like ‘宝马‘ --查等于宝马的

select * from Car where Name like ‘__E%‘ --查第三个字符是E的

% 代表是任意多个字符

_ 代表是一个字符

6.排序查询

select * from Car order by Price asc --以价格升序排列
select * from Car order by Price desc --以价格降序排列
select * from Car order by Oil desc,Price asc --以两个字段排序，前面的是主条件后面的是次要条件

7.分页查询

select top 5 * from Car
select top 5 * from Car where Code not in (select top 5 Code from Car)

当前页:page = 2; 每页显示：row = 10;

select top row * from Car where Code not in (select top (page-1)*row Code from Car)

8.去重查询

select distinct Brand from Car

9.分组查询

select Brand from Car group by Brand having count(*)>2

10.聚合函数(统计查询)

select count(*) from Car --查询所有数据条数
select count(Code) from Car --查询所有数据条数

select sum(Price) from Car --求和
select avg(Price) from Car --求平均
select max(Price) from Car --求最大值
select min(Price) from Car --求最小值

高级查询

1.连接查询

select * from Info,Nation --形成笛卡尔积

select * from Info,Nation where Info.Nation = Nation.Code

select Info.Code,Info.Name,Sex,Nation.Name,Birthday from Info,Nation where Info.Nation = Nation.Code

select * from Info join Nation on Info.Nation = Nation.Code --join on 的形式

2.联合查询

select Code,Name from Info
union
select Code,Name from Nation

3.子查询

一条SQL语句中包含两个查询，其中一个是父查询（外层查询）,另一个是子查询（里层查询）,子查询查询的结果作为父查询的条件。

--查询民族为汉族的所有人员信息
select * from Info where Nation = (select Code from Nation where Name = ‘汉族‘)

（1）无关子查询

子查询可以单独执行，子查询和父查询没有一定的关系

--查询系列是宝马5系的所有汽车信息
select * from Car where Brand =(select Brand_Code from Brand where Brand_Name = ‘宝马5系‘)

（2）相关子查询

--查找油耗低于该系列平均油耗的汽车

select * from Car where Oil<(该系列的平均油耗)
select avg(Oil) from Car where Brand = (该系列)

select * from Car a where Oil<(select avg(Oil) from Car b where b.Brand = a.Brand)