mysql> SELECT * FROM  users 

id          username             password            age         sex

1                 Tom                123                      29            0

2                 John               456                       38            0   

3                Tom                463                         27          0

4                Tom                 258                        27          0

5                Rose                369                       10            0

7               ben                    666                      8              NULL

mysql> SELECT sex FROM  users GROUP BY sex HAVING count (id) > =2;

sex

0

为什么不返回如下，当count (id) >=1才会返回如下结果，不理解这个count (id)指的什么？请老师帮忙答疑。谢谢！

sex

0

NULL