設置變量使用CASE表達式

Question

我有一個查詢，看起來像這樣：

SELECT LossCost, CoverageID     
FROM BGILossCost] 
WHERE CoverageID IN (1, 2) AND 
     StateID = @StateID AND 
     ConstructionID = @ConstructionID AND 
     PropertyClassCodeID = @PropertyClassCodeID AND 
     (BGIDistrictID = @BGIDistrictID OR BGIDistrictID IS NULL)   

，並返回下面的數據，這是正確的：

LossCost        CoverageID 
--------------------------------------- ----------- 
0.012         1 
0.034         2 

但我想要做的是設置兩個什麼取決於在CoverageID

這裏的值的變量是pseudeo代碼：

SELECT @Var1 = CASE CoverageID = 1 THEN LossCost END, 
    @Var2 = CASE CoverageID = 2 THEN LossCost END    
FROM BGILossCost] 
WHERE CoverageID IN (1, 2) AND 
     StateID = @StateID AND 
     ConstructionID = @ConstructionID AND 
     PropertyClassCodeID = @PropertyClassCodeID AND 
     (BGIDistrictID = @BGIDistrictID OR BGIDistrictID IS NULL) 

我總是可以做下面的，但我希望我也不會加入到同一個表兩次因爲性能原因

SELECT @Var1 = LossCost      
FROM BGILossCost] 
WHERE CoverageID = 1 AND 
     StateID = @StateID AND 
     ConstructionID = @ConstructionID AND 
     PropertyClassCodeID = @PropertyClassCodeID AND 
     (BGIDistrictID = @BGIDistrictID OR BGIDistrictID IS NULL) 

SELECT @Var2 = LossCost      
FROM BGILossCost] 
WHERE CoverageID = 2 AND 
     StateID = @StateID AND 
     ConstructionID = @ConstructionID AND 
     PropertyClassCodeID = @PropertyClassCodeID AND 
     (BGIDistrictID = @BGIDistrictID OR BGIDistrictID IS NULL) 

任何建議將是有益的。

Answer 1

你是非常接近的，沒有這個工作：

SELECT 
    @Var1 = MAX(CASE WHEN CoverageID = 1 THEN LossCost END), 
    @Var2 = MAX(CASE WHEN CoverageID = 2 THEN LossCost END) 
FROM BGILossCost 
WHERE CoverageID IN (1, 2) AND 
     StateID = @StateID AND 
     ConstructionID = @ConstructionID AND 
     PropertyClassCodeID = @PropertyClassCodeID AND 
     (BGIDistrictID = @BGIDistrictID OR BGIDistrictID IS NULL);