sql - sql if 语句检查是否为空

Question

我有一个县参数，如果它为空，那么我想选择协作中的所有县，如果它不为空，那么我只想要那个县的值。如何IF正确设置语句？声明的第一部分IF可能会带回多个县 ID，这就是我输入的原因WHERE CountyId。

SELECT a.CountyId, co.Description as County
FROM Application as a
    INNER JOIN Dictionary.Counties as co on a.CountyId = co.Id
WHERE co.Id in 
    IF @pCountyId is null then 
    BEGIN
        SELECT cc1.CountyId 
        FROM CountyCollaboration as cc1
        WHERE cc1.CollaborationId = (SELECT cc2.CollaborationId
                FROM CountyCollaboration as cc2
                WHERE cc2.CountyId = @pHiddenCountyId)
    END
    ELSE 
    BEGIN
        SELECT @pCountyId
    END

Answer 1

利用：

CountryId = case when @counrtyid is null then CountryId else @CounrtyId end

Answer 2

select 
    a.CountyId, 
    co.Description as County
from
    Application as a
        inner join
    Dictionary.Counties as co 
        on a.CountyId = co.Id
        inner join
    CountyCollaboration cc1
        on co.id = cc1.CountyId
        inner join
    CountyCollaboration cc2
        on cc1.CollaborationId = cc2.CollaborationId
where
    c2.CountyId = IsNull(@CountyId, @pHiddenCountyId)