—：分组查询
1 group by
select *from car group by powers;
按powers字段分组显示，去除重复项。
select *from car group by powers,oil;
先按第1个字段分组，然后在第1个字段值相同的记录中，
再根据第2个字段的值进行分组.
2 group by group_concat(字段名）
select powers,group_concat(oil) from car group by powers;
按powers字段分组后，显现每个powers字段里oil的数值。
select oil,group_concat(powers) from car group by oil;
3 group by +函数
select powers, sum(oil) from car group by powers;
按powers 字段分组，再把分组后的powers每一项的oil相加。
4 group by + with rollup
select oil,count(powers) from car group by oil with rollup;
按oil字段进行分组，再算出分组后每个oil值中powers的个数，后面一列会再最下方显示上面所有项的和
select oil, group_concat(powers) from car group by oil with rollup;
select powers,avg(price) from car group by powers with rollup;
5 group by + having + 条件
select powers,count(*) from car group by powers having count(*)>1;
按powers字段分组，数出每个项不同值的个数再挑出个数大于1的。
select powers,avg(price) from car group by powers having avg(price)>50;
二 链接查询
1 内链接
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, a.sname, b.gcourse, b.gscore from student as a inner join score as b
on a.sid = b.sid and a.sid = 3 and b.gcourse ='English';
2 左链接
select a.sid,a.sname,b.gcourse,b.gscore from student as a left join
score as b on a.sid = b.sid and a.sid = 3 and b.gcourse = 'English';
3 右链接
select a.sid,a.sname,b.gcourse,b.gscore from student as a right join
score as b on a.sid = b.sid and a.sid = 3 and b.gcourse = 'English';
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 cross join score as b;
会行程笛卡尔积
将A表的所有行分别与B表的所有行进行连接。
三 子查询
select name,price from(select *from car where powers = 130) as aa;(必须起名）
any/some
select *from student where sid > any/some(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
select * from student
union all
select *from student
把两张表拼在一起没有去重效果
union
select * from student
union
select *from score
两个表联合查询时，如果两行数据每一列都相同（无论时什么类型，只要值一样）就合并去重