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）】
]