Question

我有一张看起来像下面的表......

id           price             condition         sell
21039        20.40             new               0 
21039        20.41             used              1
12378        10.40             new               1
12378        5                 used              0
45898        30.30             new               1
45898        12.20             used              0

（注意：每个ID只会有1个新值和使用值）

我要做的是将具有相同ID号的所有行分组，但在此过程中为每个条件创建新列，这应该看起来像......

id           new_price         new_sell          used_price         new_sell
21039        20.40             0                 20.41              1
12378        10.40             1                 5                  0
45898        30.30             1                 12.20              0

我提出的所有内容都是以下查询，看起来很傻

SELECT id, price, condition, 
IF(price > 3, 1, 0)  AS sell
FROM products 
GROUP BY id

如何获得第二张桌子所需的效果。

Answer 1

这称为数据透视表。对于您需要生成的每个列，使用一系列CASE语句以及聚合MAX()或SUM()来消除NULL并将其折叠为单行。

SELECT
  id,
  SUM(CASE WHEN `condition` = 'new' THEN price ELSE 0 END) AS new_price,
  SUM(CASE WHEN `condition` = 'new' THEN sell ELSE 0 END) AS new_sell,
  SUM(CASE WHEN `condition` = 'used' THEN price ELSE 0 END) AS used_price,
  SUM(CASE WHEN `condition` = 'used' THEN sell ELSE 0 END) AS used_sell
FROM 
  products
GROUP BY id

如果没有SUM()和GROUP BY，您仍会获得每id行2行，每行有一半的列（condition中不匹配CASE } {} NULL。 SUM()（在这种情况下也可以使用MAX()）消除NULL并生成一行，因为聚合函数排除NULL值，而GROUP BY将行按id分组。 / p> SQLFiddle.com上的

Here is a working sample

评论后更新：

要根据sell计算price，只需替换sell CASE语句中的条件：

SELECT
  id,
  SUM(CASE WHEN `condition` = 'new' THEN price ELSE 0 END) AS new_price,
  SUM(CASE WHEN `condition` = 'new' AND price > 3 THEN 1 ELSE 0 END) AS new_sell,
  SUM(CASE WHEN `condition` = 'used' THEN price ELSE 0 END) AS used_price,
  SUM(CASE WHEN `condition` = 'used' AND price > 3 THEN 1 ELSE 0 END) AS used_sell
FROM 
  products
GROUP BY id

（Updated sample...）

按ID分组结果并创建新列

1 个答案:

Here is a working sample

评论后更新：