GROUP_CONCAT()
函数的值等于属于一个组的指定列的所有值，以逗号隔开，并且以字符串表示
mysql> select sex,group_concat(level) from role group by sex;
+------+---------------------+
| sex | group_concat(level) |
+------+---------------------+
| 0 | 8,4,91 |
| 1 | 10,20,20,29,60 |
+------+---------------------+
2 rows in set (0.00 sec)
with rollup子句：用来要求在一条group by子句中进行多个不同的分组
mysql> select sex,count(1) from role group by sex with rollup;
+------+----------+
| sex | count(1) |
+------+----------+
| 0 | 3 |
| 1 | 5 |
| NULL | 8 |
+------+----------+
3 rows in set (0.00 sec)
