关于MySQL的几个函数

find_in_set

+----+-----------+
| id | extra_ids |
+----+-----------+
| 1  | 1,2,3     |
| 2  | 4,5,6,1,3 |
| 3  | 7,3,1,2,5 |
+----+-----------+

-- 看到有人是用like做的，虽然这两种都会全表扫描，这样用会比like好。
select * from test where find_in_set(5, extra_ids);

group_concat

+----+----------+
| id | group_id |
+----+----------+
| 1  | 1        |
| 2  | 1        |
| 3  | 2        |
+----+----------+

-- group_concat(id)
select group_concat(id order by id desc separator '|') as ids from test group by group_id;

+-----+----------+
| ids | group_id |
+-----+----------+
| 2|1 | 1        |
| 3   | 2        |
+-----+----------+