用SQL对经过排名的结果集进行转置

Posted 2023-02-27 wzy0623

问题

        想对表中的值进行排名，然后将结果集转置为 3 列。这样做旨在分别显示前 3 名、接下来的 3 名以及其余各行记录。例如，表中记录如下：

mysql> select * from t1;
+------+
| a    |
+------+
| 5000 |
| 2850 |
| 1500 |
| 3000 |
| 2450 |
| 1300 |
| 3000 |
| 1600 |
| 1250 |
| 2975 |
| 1250 |
| 1100 |
|  950 |
|  800 |
+------+
14 rows in set (0.00 sec)

        想根据 a 进行排名，然后将结果转置为 3 列，以得到如下结果集：

+-------+--------+------+
| TOP_3 | NEXT_3 | REST |
+-------+--------+------+
|  5000 |   2850 | 1500 |
|  3000 |   2450 | 1300 |
|  3000 |   1600 | 1250 |
|  2975 |        | 1250 |
|       |        | 1100 |
|       |        |  950 |
|       |        |  800 |
+-------+--------+------+

实现：

select max(case when f=1 then a end) TOP_3,
       max(case when f=2 then a end) NEXT_3,
       max(case when f=3 then a end) REST
  from (select a,
               case when rn<=3 then 1 when rn<=6 then 2 else 3 end f,
               row_number() over (partition by case when rn<=3 then 1 when rn<=6 then 2 else 3 end) rn
          from (select a,dense_rank() over (order by a desc) rn from t1) t) t 
 group by rn;