mysql - 将多个查询合并为一个

Question

是否可以将多个查询合并为一个？

我有 2 个:

SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
FROM Table
WHERE Color = 'Red'

和

SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
FROM Table
WHERE Color = 'Blue'

现在我想在一个查询中同时进行

也可以这样尝试:

SELECT
(
SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
FROM Table
WHERE Color = 'Red'
)
(
SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff2
FROM Table
WHERE Color = 'Blue'
)

但确实有效 :(

Answer 1

您可以使用带有 IN 运算符的单个查询。这是多个 OR 条件的缩写:

SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
FROM Table
WHERE Color IN ( 'Red', 'Blue' )

如果你真的坚持(气馁)有两个查询，然后使用 UNION ALL 来组合它们:

SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
FROM Table
WHERE Color = 'Red'
UNION ALL -- does not remove duplicates from output
SELECT DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
FROM Table
WHERE Color = 'Blue'

如果要删除重复项，请使用 UNION 而不是 UNION ALL。

---

如果您需要在评论中提到的不同列中产生结果，则为此使用 CASE 语句。不过，我真的没有看到这样做的原因。

SELECT 
  CASE WHEN Color = 'Red' THEN DATEDIFF(CURRENT_DATE, cl.updated_at) END AS Diff,
  CASE WHEN Color = 'Blue' THEN DATEDIFF(CURRENT_DATE, cl.updated_at) END AS Diff2,
FROM Table
WHERE Color IN ( 'Red', 'Blue' )

对于第二种方法:

SELECT
  CASE WHEN Color = 'Red'  THEN Diff END AS Diff,
  CASE WHEN Color = 'Blue' THEN Diff END AS Diff2
FROM (
  SELECT Color, DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
  FROM Table
  WHERE Color = 'Red'
  UNION ALL -- does not remove duplicates from output
  SELECT Color, DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff
  FROM Table
  WHERE Color = 'Blue'
) t