子查詢中CASE subselecting時返回超過1個行時聲明

Question

首先查詢

此查詢工作正常，發動機不抱怨

SELECT id 
FROM agencies 
WHERE id IN ((SELECT id FROM agencies)) 
ORDER BY id; 

第二個查詢

這一個不工作，發動機正在抱怨Subquery returns more than 1 row，當 - 根據我 - 我正在做同樣的事情@param_1 IS NULL

SELECT 
    @param_1 := NULL, 

SELECT id 
FROM agencies 
WHERE id IN (CASE WHEN @param_1 IS NULL THEN (SELECT id FROM agencies) ELSE 1 END) 
ORDER BY id; 

有沒有人看到爲什麼引擎在第一次查詢時沒有抱怨第二個查詢？

在此先感謝，

Answer 1

CASE需要一個標量單值。不是一個記錄集。

SELECT id 
FROM agencies 
WHERE id IN (
     SELECT id FROM agencies WHERE @param_1 IS NULL 
     UNION ALL 
     SELECT 1 WHERE @param_1 IS NOT NULL 
     ) 
ORDER BY id; 

OR

SELECT id 
FROM agencies 
WHERE id IN (SELECT id FROM agencies) 
    AND @param_1 IS NULL 
UNION ALL 
SELECT id 
FROM agencies 
WHERE @param_1 IS NOT NULL AND id = 1 
ORDER BY id; 

另一種選擇是使用IF

IF @param_1 IS NULL 
    SELECT id 
    FROM agencies 
    WHERE id IN (SELECT id FROM agencies) 
    ORDER BY id; 
ELSE 
    SELECT id 
    FROM agencies 
    WHERE id = 1 
    ORDER BY id; 

Answer 2

更改爲：

SELECT id 
FROM agencies 
WHERE id IN (
       SELECT id 
       FROM agencies 
       WHERE @param_1 IS NULL 

       UNION ALL 

       SELECT 1 as id 
       WHERE @param_1 IS NOT NULL 
) 
ORDER BY id;