高级查询

一、分组查询

1、group by：

　　select * from car group by powers；

2、group by group_concat(字段名)   ：

select powers,group_concat(name) from car group by powers;

3、group by +函数；

4、group by = with rollup；

select oil,powers from car group by powers with rollup; 

5、group by + having + 条件：

select powers,count(*) from car group by powers having count(*)>1;

select powers,avg(price) from car group by powers having avg(price)>50;

二、链接查询

1、内连接：

select a.sid，sname，gcourse，gscore from student as a，score as b

where a.sid = b sid and a.sid = 3 and b.gcourse = "chinese";

select a.sid，sname，gcourse，gscore from student as a,inner join score

as b where/on a.sid = b.sid and a.sid = 3 and b.sid = "chinese";

 2、左链接：

select a.sid,sname,gcourse,gscore from student as a left join score as b on a.sid = b.sid

and a.sid = 3 and b.gcourse = "chinese";　　

3、右链接：

select a.sid,sname,gcourse,gscore from student as a right join score as b on a.sid = b.sid

and a.sid = 3 and b.gcourse = "chinese";　　

4、自然链接：

select * from student as a natural join score as b

自动清除笛卡尔积，把重复的列合并

5、交叉链接：

select * from student as a corss  join score as b；

会形成笛卡尔积

三、子查询：

select name price from (select * from car where powers =130) as aa;

[必须起别的名字]

any/more：

select * from student where sid > any/more(select gid from score)

all:

select * from student where sid > all (select gid from score)

exists:

select * from student where exists (select sid from score where sid = 88)

【因为score表中sid没有等于88的  所以 导致外层查询不执行】

select * from student where sid = 1 and exists (select sid from score where sid = 1)

select * from student where exists (子查询是否返回有结果集，如果有，则执行外层查询，

如果没有，则不执行外层查询)

 四、联合查询：查询时，查询的字段个数要一样   

    union all  

例如      select *from student union all select * from score     

                【把两个表拼接在一起，没有去重效果】

      uniom  

例如       select *from student union select *from socer

         【两个表联合在一起查询时,如果两行数据每一列相同（无论是什么类型只要值一样，就会合并去重）】

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