基本查询
select * from aa;
+------+------+| id| name |+------+------+|1 | 10||1 | 20||1 | 20||2 | 20||3 | 200 ||3 | 500 |+------+------+6 rows in set (0.00 sec)
以id分组，把name字段的值打印在一行，逗号分隔(默认)
select id,group_concat(name) from aa group by id;
