test表:
A B C
1 a y
2 b n
3 c y
4 d n
=============
trymysql>select B,count(A) as cc,sum(C = 'y') as yc from test group by B
A B C
1 a y
2 b n
3 c y
4 d n
=============
trymysql>select B,count(A) as cc,sum(C = 'y') as yc from test group by B
但是,为什么必须使用SUM(),而不能使用COUNT()呢??
----------------------------------------------------
mysql> select B,C='y' from test group by B;
+------+-------+
| B | C='y' |
+------+-------+
| a | 1 |
| b | 0 |
| c | 1 |
| d | 0 |
+------+-------+
4 rows in set (0.00 sec)