SQL server检查空值

我有一个表，我正在添加一个约束，因此如果PaymentType不同于“Check”，请确保CheckNumber为空。这是我的桌子：

create table RegistrationHeader(
    RegistrationNo numeric 
    ,BillingID varchar(30) unique not null
    ,RegistrationDate date not null
    ,PaymentType varchar(5) check (PaymentType = 'CC' or PaymentType = 'PO' or PaymentType = 'Check') not null
    ,CCNumber varchar(16) check(LEN(CCNumber) = 16 OR LEN(CCNumber) = 15)
    ,PONumber varchar(30)
    ,CheckNumber varchar(10) default null
    ,primary key(RegistrationNo)
    ,constraint CC_CCNumber_constr check(
        (PaymentType = 'CC' and CCNumber is not null)
        or
        (PaymentType != 'CC' and CCNumber is null)
    )
    ,constraint PO_PONumber_constr check(
        (PaymentType = 'PO' and (PONumber is not null or PONumber != ''))
        or
        (PaymentType != 'PO' and PONumber is null)
    )
);

所以，我试过了

,constraint CheckNumber_type_constr check((PaymentType = 'CC' or PaymentType = 'PO') and CheckNumber is null)

及

两人都惨遭失败

那么：

constraint CheckNumber_type_constr
    check ((PaymentType = 'Check') or (CheckNumber is null))

如果要确保检查的

CheckNumber

也不为空：

constraint CheckNumber_type_constr
    check ((PaymentType = 'Check' and CheckNumber is not null) or
           (PaymentType <> 'Check' and CheckNumber is null)
          )

constraint CheckNumber\u type\u constr
支票（（PaymentType='check'且支票编号不为空）或
（PaymentType“Check”和CheckNumber为空）
)

constraint CheckNumber_type_constr
    check ((PaymentType = 'Check' and CheckNumber is not null) or
           (PaymentType <> 'Check' and CheckNumber is null)
          )