如何使用內部Where子句中SQL

Question

我在數據庫視圖有這個元素視圖元素：

CASE 
    WHEN Type = 'Reserved' THEN CAST(1 AS bit) 
    ELSE CAST(0 AS bit) 
    END AS isReserved, 

我想用我的WHERE子句中的列isReserved。我怎樣才能做到這一點？謝謝。

Answer 1

您可以使用CROSS APPLY ... VALUES（在SQL Server - 沒有注意到它沒有標記爲任何特定的DBMS）

SELECT isReserved 
FROM YourTable 
     CROSS APPLY (VALUES(CASE WHEN Type = 'Reserved' THEN CAST(1 AS BIT) 
          ELSE CAST(0 AS BIT) 
          END)) CA(isReserved) 
WHERE isReserved = 1 

但你最好直接在你的WHERE子句中使用Type = 'Reserved'等，而不是直接引用CASE結果。這樣就可以使用索引。

Answer 2

你不能直接做。

要麼包含子查詢。

SELECT * 
FROM (SELECT <something> as newAlias FROM YourTable) as T 
WHERE T.newAlias = .... 

或重複剛纔的定義

SELECT * 
FROM YourTable 
WHERE <something> = .... 

Answer 3

您可以使用WHERE子句，和的Oracle SQL不會再計算它一樣的表情，它會被從緩存中

select CASE 
    WHEN Type = 'Reserved' THEN CAST(1 AS bit) 
    ELSE CAST(0 AS bit) 
    END AS isReserved 
from tablename 
where CASE 
    WHEN Type = 'Reserved' THEN CAST(1 AS bit) 
    ELSE CAST(0 AS bit) 
    END=CAST(0 AS bit)