Where子句中的SQL Server条件

我正在上SQL Server课程。存储过程中的条件Where子句有问题。3个参数中有2个可能为空

@Service nvarchar(50),
@Country nvarchar(50)  = NULL,
@Region nvarchar(50) = NULL

as 

SELECT
  CASE @Service
    WHEN 'Army' THEN Sum(Army)
    WHEN 'Navy' THEN Sum(Navy)
    When 'Marine_Corps' then Sum(Marine_Corps)
    When 'Air_Force' then Sum(Air_Force)
                ELSE NULL
  END

  as "ServiceTotal"

FROM
  All_Records

 WHERE 

if @Country is not null @Country and if @Region !=Null @Region = Region

如何重新构造Where子句？我来自一个访问背景

GetTotals Navy

OR

GetTotals Navy,Albania,Europe

我想强制提供服务，国家和地区可选。所以我希望能够像这样调用这个过程。所以，如果我没有指定地区和国家，它应该返回所有海军

---

这可能吗？

就性能而言，最好不要在

WHERE

子句中使用函数

SELECT  CASE @Service
            WHEN 'Army' THEN Sum(Army)
            WHEN 'Navy' THEN Sum(Navy)
            When 'Marine_Corps' then Sum(Marine_Corps)
            When 'Air_Force' then Sum(Air_Force)
            ELSE NULL
        END AS "ServiceTotal"
FROM All_Records
WHERE Country = CASE WHEN @Country IS NOT NULL THEN @Country ELSE Country END
AND Region = CASE WHEN @Region IS NOT NULL THEN @Region ELSE Region END

使用

ISNULL

函数

SELECT  CASE @Service
            WHEN 'Army' THEN Sum(Army)
            WHEN 'Navy' THEN Sum(Navy)
            When 'Marine_Corps' then Sum(Marine_Corps)
            When 'Air_Force' then Sum(Air_Force)
            ELSE NULL
        END AS "ServiceTotal"
FROM All_Records
WHERE Country = ISNULL(@Country ,Country)
AND Region = ISNULL(@Region ,Country)

试试这个：

Where (@country is null or @country = country)
And (@region is null or @region = region)

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