MySQL查询 - 悦读

1 基本查询

select 字段列表 from  表名
	基本查询
	select * from person 

	SELECT id,name,age,sex,favourite from person

2 条件查询

用where表示查询的条件
	SELECT id,name,age ,sex ,favourite  from person where id BETWEEN 1 and 10
	SELECT id,name,age ,sex ,favourite  from person where  id>5 and sex='男'   
	SELECT id,name,age ,sex ,favourite  from person where  id>5 or sex='男'    
%匹配0或多个字符，一般不用左模糊（%放在左边，导致索引失效，降低查询效率）
	SELECT id,name,age ,sex ,favourite  from person where name like '%豪'
_代表匹配一个字符
	SELECT id,name,age ,sex ,favourite  from person where name like '_豪'
	SELECT id,name,age ,sex ,favourite  from person where name like '%二%'

3 分页查询

语法 limit 起始索引，个数
分页查询第m页，每页展示n 条数据 limit (m-1)*n, n 
SELECT id,name,age ,sex ,favourite  from person limit 0,5
SELECT id,name,age ,sex ,favourite  from person limit 5,5
SELECT id,name,age ,sex ,favourite  from person limit 10,5

4 排序

排序 order by 字段名 （ASC 升序，DESC降序）
SELECT id,name,age ,sex ,favourite from person ORDER BY age ASC;
SELECT id,name,age ,sex ,favourite from person ORDER BY age DESC;

5 聚合函数

聚合函数  count()   sum()  avg()   max()  min()
select count(0) from person   查询有多少条记录
select sum(age) from person 
select avg(age) from person 
select min(age) from person 
select max(age) from person 
SELECT CAST(avg(age) as decimal(10,0)) as avg from person

6 分组查询

分组查询  group by 字段
SELECT  sex,count(0),avg(age) from person GROUP BY sex;

having条件查询  where分组前，having分组后的条件
SELECT  sex,count(0),avg(age) from person where age>8 GROUP BY sex  having avg(age)>20;

7 子查询

把查询到的结果当作另一个查询的条件
SELECT id,name,age ,sex ,favourite  from person where id in (SELECT pid from dept where name='研发部')

8 内连接

笛卡尔积 查询出来的结果没有意义
select * from person,dept

显示内连接 inner可以省略
select * from person INNER JOIN dept on person.id=dept.pid
select * from person  JOIN dept on person.id=dept.pid
select p.id,p.name,age,sex,favourite,birth,d.id deptid,d.name deptname,pid from person p  JOIN dept d on p.id=d.pid

隐式内连接
select * from person,dept where person.id=dept.pid

9 外连接

左外连接  left [outer] join  查询的是左边表的所有部分和右边表的交集
select p.id,p.name,age,sex,favourite,birth,d.id  部门id,d.name as deptname,pid from person p left outer join dept d on p.id=d.pid
select p.id,p.name,age,sex,favourite,birth,d.id  部门id,d.name as deptname,pid from person p left  join dept d on p.id=d.pid
右外连接  right [outer] join  查询的是右边表的所有部分和左边表的交集
select p.id,p.name,age,sex,favourite,birth,d.id  部门id,d.name as deptname,pid from person p right outer join dept d on p.id=d.pid