Question

如何优化此代码可以在O（n）中运行，以便在@TollPrice中赋值：

IF (EXISTS (SELECT TollPrice
            FROM Car_TaxInfo
            WHERE (car_subgrp_id = @Kind) AND (Sal = @Sal)))
    BEGIN
        SELECT  @TollPrice = TollPrice
        FROM   Car_TaxInfo
        WHERE     (car_subgrp_id = @Kind) AND (Sal = @Sal)
        SET @IsExistToll = 1
    END
ELSE
    BEGIN
        SET @IsExistToll = 0
    END

Answer 1

您无需在此验证是否存在：

SET @TollPrice = NULL --this is mandatory. If @TollPrice contains some value then it will retain that value after below statement if there will be no matching rows.

SELECT  @TollPrice = TollPrice
FROM    Car_TaxInfo
WHERE   (car_subgrp_id = @Kind) AND (Sal = @Sal)

IF @TollPrice IS NOT NULL
   SET @IsExistToll = 1
ELSE
   SET @IsExistToll = 0

如果TollPrice本身可以NULL，那么您可以使用@@ROWCOUNT

SELECT  @TollPrice = TollPrice
FROM    Car_TaxInfo
WHERE   (car_subgrp_id = @Kind) AND (Sal = @Sal)

IF @@ROWCOUNT > 0
   SET @IsExistToll = 1
ELSE
   SET @IsExistToll = 0

更重要的是，您可以执行以下操作：

SET @IsExistToll = 0

SELECT  @TollPrice = TollPrice, @IsExistToll = 1
FROM    Car_TaxInfo
WHERE   (car_subgrp_id = @Kind) AND (Sal = @Sal)

Answer 2

您的系统是OLAP系统还是PLTP系统？如果是OLAP或OLTP具有可接受的INSERT / UPDATE / Delete数量，则可能需要为car_subgrp_id和Sal列添加索引。在IF select子句中，您可以将查询与TOP 1组合，并将EXISTS替换为此新查询。我的意思是你的新查询可能如下所示：

Declare @tPrice INT
IF (SELECT TOP 1 @tPrice=TollPrice
        FROM Car_TaxInfo
        WHERE (car_subgrp_id = @Kind) AND (Sal = @Sal) IS NOT NULL)
    SET @IsExistToll = 1
ELSE
    SET @IsExistToll = 0

如何优化此SQL查询

2 个答案: