（行到列/ Pivot）+ SUM

Question

我在MySQL中有一些表：

+---------+--------------+---------+    +---------+----------+
|             TABLE 1              |    |       TABLE 2      |
+---------+--------------+---------+    +---------+----------+
| id_user |  testnumber1 |   score |    | id_user | username |
+---------+--------------+---------+    +---------+----------+
|    1    |       1      |    90   |    |    1    |   aaa    |
|    1    |       2      |    80   |    |    2    |   bbb    |
|    1    |       3      |    70   |    |    3    |   ccc    |
|    2    |       1      |    60   |    +---------+----------+
|    2    |       2      |    66   |
|    2    |       3      |    77   |
|    3    |       1      |    90   |
|    3    |       2      |    80   |
|    3    |       3      |    70   |
+---------+--------------+---------+

有可能得到这个：

+---------+--------------+--------------+--------------+
|                      TABLE RESULT                    |
+---------+--------------+--------------+--------------+ 
| id_user |  testnumber1 |  testnumber2 |  testnumber3 |
+---------+--------------+--------------+--------------+
|    1    |      90      |      80      |      70      |
|    2    |      60      |      66      |      77      |
|    3    |      90      |      80      |      70      |
+---------+--------------+--------------+--------------+

我可以使用什么声明来获得此结果？

Answer 1

您需要将PIVOT列添加到行中。不幸的是，MySQL没有PIVOT表运算符。但您可以使用CASE表达式执行此操作，如下所示：

SELECT
  id_user,
  MAX(CASE WHEN  testnumber1 = 1 THEN score END) AS testnumber1,
  MAX(CASE WHEN  testnumber1 = 2 THEN score END) AS testnumber2,
  MAX(CASE WHEN  testnumber1 = 3 THEN score END) AS testnumber3
FROM Table
GROUP BY id_user;

SQL Fiddle Demo