select count(*) from ****** a left join **** group by a._,a._ a._ ……
查询出来的结果是
怎么实现为 结果 是
如表为test,数据如下:
id grade
1 100
1 80
2 90
2 80
3 90
此时,可用count可求每个id在表中的个数,可用如下sql语句
select id,count(*) as counts from test group by id;结果如下:
id counts
1 2
2 2
3 1
但group by还用另一种用法,就是配合having来使用,如表数据不变,现在要求在表中,id出现大于一次的id及个数
select id,count(*) as counts from test group by id having count(*)>1;结果就是:
id counts
1 2
2 2