select * from user;
+------+------+-----+
| id   | name | key |
+------+------+-----+
| 1000 | 张三 |   1 |
| 1001 | 李四 |   2 |
| 1002 | 刘五 |   3 |
+------+------+-----+

select * from map;
+-----+-------+
| key | value |
+-----+-------+
|   1 | aaa   |
|   2 | bbb   |
|   3 | ccc   |
+-----+-------+

select *, u.key as uk, m.key as mk from user u left join map m on u.key = m.key;
+------+------+-----+-----+-------+----+----+
| id   | name | key | key | value | uk | mk |
+------+------+-----+-----+-------+----+----+
| 1000 | 张三 |   1 |   1 | aaa   |  1 |  1 |
| 1001 | 李四 |   2 |   2 | bbb   |  2 |  2 |
| 1002 | 刘五 |   3 |   3 | ccc   |  3 |  3 |
+------+------+-----+-----+-------+----+----+