基於兩個表關係的約束？

Question

基本上我只想在表B中的列爲NULL時允許表A中的編輯/更新。表A和表B引用相同的主鍵ID。所以像是;

-- Only allow Table A to be updated when Table B column is NULL 

update TableA 
set blah = @a, foo = @b 
from tablea 
inner join tableb on tablea.id = tableb.id 
where tableb.column is null 

是否有某種方法可以將此用作表A的約束條件？

Answer 1

CREATE TRIGGER tr_Instead_Update_Table_A 
ON TABLEA 
INSTEAD OF UPDATE 
AS 
BEGIN 
    SET NOCOUNT ON; 

    IF NOT EXISTS(SELECT 1 
       FROM deleted d INNER JOIN TableB B 
       ON d.PK_Column = B.PK_Column 
       WHERE B.ColumnName IS NULL) 
    BEGIN 
     RAISERROR('Invalid update',16,1) 
     RETURN; 
    END 

    -- Your update statement here 

END