mysql> select sum(distinct id), sum(distinct int_col) from alltypesagg;
+--------------------+-------------------------+
| sum(DISTINCT id) | sum(DISTINCT int_col) |
+--------------------+-------------------------+
| 0 | 0 |
+--------------------+-------------------------+
1 row in set (0.01 sec)

mysql> select sum(distinct id) from alltypesagg;
+--------------------+
| sum(DISTINCT id) |
+--------------------+
| NULL |
+--------------------+
1 row in set (0.01 sec)

当表中没有数据时，sum distinct应该返回NULL， 而不是0，因为0对sum聚合函数来说是有意义的数。单个sum distinct计算结果没有问题，但是多个sum distinct的计算结果有问题。