SQL中的where时的大小写

这就是你想做的吗

DECLARE @ONum int --variable
DECLARE @CrUsId nvarchar(128) = 'D901D15C-62FA-4243-A3DB-D3D448DC3F91'--assign value

DECLARE @From Datetime = 4/2/2017  --assign value
DECLARE @To Datetime  = 25/2/2017  --assign value

`declare @day nvarchar(50)`
`select * from PurchaseOrder`

where 
 CreateUserID =  CASE @CrUsId
  WHEN @CrUsId = '' THEN CreateUserID
 WHEN @CrUsId <> CreateUserID THEN NULL  -- if input is some text but      
   WHEN @CrUsId IS NULL THEN CreateUserID
  WHEN @CrUsId != '' AND CreateUserID = @CrUsId THEN CreateUserID
 END

---

你有问题吗？您的代码看起来很糟糕，但这不是问题。这是非标准SQL。您使用的是哪种数据库管理系统？

select * from PurchaseOrder
    where CreateUserID =  
        CASE 
            WHEN @CrUsId = '' OR @CrUsId IS NULL THEN CreateUserID
            WHEN @CrUsId <> CreateUserID THEN NULL  -- if input is some text but      
            WHEN @CrUsId = CreateUserID THEN CreateUserID
        END