同一张表 COUNT GROUP BY 和 select 如何合并成一条语句？

Question

tb 表的结构：

a	b
1	2
1	2
1	3

我想得到如下数据：

a	b	num(a,b列相同的行数)
1	2	2
1	2	2
1	3	1

SELECT COUNT(*) as num FROM tb GROUP BY a,b

SELECT * from tb

请问如何用一句sql写出来呢？

Answer 1

select a,b,count(*)as count from test group by a,b

select t1.a,t1.b,t2.count from test as t1 left join (select a,b,count(*)as count from test group by a,b) as t2 on t1.a = t2.a and t1.b = t2.b

Answer 2

手机码了一下，使用子查询。
select t1.a, t1.b, t2.num
from
(
select a, b from tb
) as t1
left join
(
select a, b, count(1) as num from tb group by a,b
) as t2 on t2.a = t1.a and t2.b = t1.b
;