postgre如何在SQL中不使用pivot函数的情况下进行透视

Question

我想在不使用任何PIVOT函数的情况下透视表，但只使用纯SQL(例如PostgreSQL语法(。

以这个数据集为例：

Order |   Element | Value |
---------------------------
1     | Item      | Bread |
1     | Quantity  |  3    |
1     | TotalCost |  3,30 |
2     | Item      | Pizza |
2     | Quantity  |  2    |
2     | TotalCost |  10   |
3     | Item      | Pasta |
3     | Quantity  |  5    |
3     | TotalCost |  2,50 |

我希望以Order专栏为中心，并有一些类似的内容：

Order | Item | Quantity | TotalCost |
-------------------------------------
1     | Bread |    3    |    3,30   |
2     | Pizza |    2    |    10     |
3     | Pasta |    5    |    2,50   |

如果不使用任何枢轴功能，我将如何再次实现这一点？

Answer 1

解决方案A

使用CASE WHEN语句：

SELECT Order, 
MAX(CASE WHEN Element = 'Item' THEN Value END) AS Item, 
MAX(CASE WHEN Element = 'Quantity' THEN Value END) AS Quantity, 
MAX(CASE WHEN Element = 'TotalCost' THEN Value END) AS TotalCost
FROM MyTable
GROUP BY 1

解决方案B

使用自左联接：

SELECT A.Order, A.Item, B.Quantity, C.TotalCost
FROM
(SELECT Order, Value as Item
FROM MyTable
WHERE Element = 'Item') as A
LEFT JOIN
(SELECT Order, Value as Quantity
FROM MyTable
WHERE Element = 'Quantity') as B ON A.Order = B.Order
LEFT JOIN
(SELECT Order, Value as TotalCost
FROM MyTable
WHERE Element = 'TotalCost') as C ON B.Order = C.Order

后者可能没有那么有效，但在某些用例中可能会有用。

Answer 2

在Postgres中，您将使用FILTER:

SELECT Order, 
MAX(Value) FILTER (WHERE Element = 'Item') AS Item, 
MAX(Value) FILTER (WHERE Element = 'Quantity') AS Quantity, 
MAX(Value) FILTER (WHERE Element = 'TotalCost') AS TotalCost
FROM MyTable
GROUP BY 1;

请注意，ORDER是一个SQL关键字，因此对于列名来说，这是一个错误的选择。