將多個查詢合並為一個

Question

是否可以將多個查詢合而為一？

我有2個：

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'

現在我要兩個查詢

所以是這樣嘗試：

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'
)

但是要做好工作:(

Answer 1

您可以使用帶有IN運算符的單個查詢。 這是多個OR條件的縮寫：

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

如果您真的堅持（勸阻）使用兩個查詢，則可以使用UNION ALL組合它們：

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'

如果要刪除重復項，請使用UNION而不是UNION ALL 。

---

如果您需要在注釋中提到不同的結果列，請為此使用CASE語句。 不過，我確實沒有找到原因。

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' )

對於第二種方法：

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

Answer 2

在這種情況下，它不只是更新where嗎？

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

是的，您可以使用INNER JOIN或UNION，但是對於此示例，這將無用。

Answer 3

您可以使用UNION運算符來聯接兩個查詢。 https://www.w3schools.com/sql/sql_union.asp

Answer 4

如果所有查詢中的輸出列都相同，則可以使用UNION / UNION ALL如下所示：

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

UNION

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

Answer 5

如果可以使用不同行中的值，則只需執行以下操作：

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

我將Color添加到SELECT以便您可以區分行。

如果子查詢返回一個值，則您只是缺少逗號：

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'
       );

如果子查詢返回多行，則需要解釋所需的結果集。

編輯：

如果您希望這些值在兩個不同的列中對齊，則可以執行以下操作：

select max(diff_red) as diff_red, max(diff_blue) as diff_blue
from ((select (@rnr := @rnr + 1) as rn, 
              DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff_red,
              NULL AS Diff_blue
       from t cross join
            (select @rnr := 0) params
       where color = 'Red'
      ) union all
      (select (@rnb := @rnb + 1) as rn, 
              NULL AS Diff_red,
              DATEDIFF(CURRENT_DATE, cl.updated_at) AS Diff_blue
       from t cross join
            (select @rnrb:= 0) params
       where color = 'Blue'
      )
     ) rb
group by rn;