MySQL，多行来分隔字段

Question

我有一个MySQL表，其中包含以下字段和数据;

PartNumber  Priority SupName
a1            0        One
a2            0        One
a2            1        Two
a3            0        One
a4            1        Two
a5            2        Three

我正在尝试创建一个视图，其中有多行的部分组合成一行，并分成单独的字段，例如

理想情况下;

PartNumber  Sup1  Sup2  Sup3
a1          One   NULL  NULL
a2          One   Two   NULL
a3          One   NULL  NULL
a4          Two   NULL  NULL
a5          Three NULL  NULL

或者我可以忍受这个

PartNumber  Sup1  Sup2  Sup3
a1          One   NULL  NULL
a2          One   Two   NULL
a3          One   NULL  NULL
a4          NULL  Two   NULL
a5          NULL  NULL  Three

我如何构建一个视图或select语句来完成此任务？

我到目前为止最接近的是;

SELECT PartNumber, 
       IF(Priority=0, SupName, NULL) AS Sup1, 
       IF(Priority=1, SupName, NULL) AS Sup2,
       IF(Priority=2, SupName, NULL) AS Sup3 
  FROM SupXref 
ORDER BY PartNumber

然而，这为每个字段提供了一个单独的行，我需要一行。

Answer 1

你只是错过了一个小组：）

SELECT PartNumber,
  MAX(IF (Priority = 0, SupName, NULL)) AS Sup1,
  MAX(IF (Priority = 1, SupName, NULL)) AS Sup2,
  MAX(IF (Priority = 2, SupName, NULL)) AS Sup3
FROM SupXref
GROUP BY PartNumber

修改

玩了一段时间后，我想我找到了你想要的第一个解决方案。试一试：）

SELECT partnumber,
  COALESCE(Sup1, COALESCE(Sup2, Sup3)) AS Supp1,
  IF (Sup1 IS NULL, IF (Sup2 IS NULL, NULL, Sup3), COALESCE(Sup2, Sup3)) AS Supp2,
  IF (Sup1 IS NULL, NULL, IF (Sup2 IS NULL, NULL, Sup3)) AS Supp3
FROM (
  SELECT PartNumber,
    MAX(IF (Priority = 0, SupName, NULL)) AS Sup1,
    MAX(IF (Priority = 1, SupName, NULL)) AS Sup2,
    MAX(IF (Priority = 2, SupName, NULL)) AS Sup3
  FROM SupXref
  GROUP BY PartNumber
) AS S

对于下表：

+------------+----------+---------+
| PARTNUMBER | PRIORITY | SUPNAME |
+------------+----------+---------+
| a1         |        2 | Three   |
| a2         |        1 | Two     |
| a3         |        2 | Three   |
| a3         |        1 | Two     |
| a4         |        0 | One     |
| a5         |        0 | One     |
| a5         |        2 | Three   |
| a6         |        0 | One     |
| a6         |        1 | Two     |
| a7         |        0 | One     |
| a7         |        1 | Two     |
| a7         |        2 | Three   |
+------------+----------+---------+

数据转变为：

+------------+------+------+-------+
| PARTNUMBER | SUP1 | SUP2 | SUP3  |
+------------+------+------+-------+
| a1         |      |      | Three |
| a2         |      | Two  |       |
| a3         |      | Two  | Three |
| a4         | One  |      |       |
| a5         | One  |      | Three |
| a6         | One  | Two  |       |
| a7         | One  | Two  | Three |
+------------+------+------+-------+

最后进入这个：

+------------+-------+-------+-------+
| PARTNUMBER | SUPP1 | SUPP2 | SUPP3 |
+------------+-------+-------+-------+
| a1         | Three |       |       |
| a2         | Two   |       |       |
| a3         | Two   | Three |       |
| a4         | One   |       |       |
| a5         | One   | Three |       |
| a6         | One   | Two   |       |
| a7         | One   | Two   | Three |
+------------+-------+-------+-------+