Check Constraint – only allow one column to be true if another column is true

Question

Take the following example table:

CREATE TABLE [dbo].[tbl_Example](
    [PageID] [int] IDENTITY(1,1) NOT NULL,
    [RequireLogin] [bit] NOT NULL,
    [RequireAdmin] [bit] NOT NULL,
    [HideIfLoggedIn] [bit] NOT NULL
)

How would one rewrite the above to include check constraints as follows:

• Force [RequireAdmin] to be False if [RequireLogin] is False (i.e only allow [RequireAdmin] to be True if [RequireLogin] is True whilst allowing [RequireLogin] to be True and [RequireAdmin] to be False
• Only allow [HideIfLoggedIn] to be True if [RequireLogin] is False

Answer 1

You typically do nested case statements in the check in order to get that type of logic to work. Remember that a case in a check must still be an evaluation, so it would take the form

CHECK (case when <exp> then 1 end = 1).

Looking over your exact requirements however it seems that this would also work and is probably easier to read:

 CREATE TABLE [dbo].[tbl_Example]
 (
    [PageID] [int] IDENTITY(1,1) NOT NULL,
    [RequireLogin] [bit] NOT NULL,
    [RequireAdmin] [bit] NOT NULL,
    [HideIfLoggedIn] [bit] NOT NULL
 )

 ALTER TABLE [dbo].[tbl_Example] ADD CONSTRAINT
     [RequireAdmin] CHECK 
      ((RequireAdmin = RequireLogin) OR 
      (RequireLogin=1));

 ALTER TABLE [dbo].[tbl_Example] ADD CONSTRAINT
      [HideIfLoggedIn] CHECK 
      ((RequireLogin=1 AND HideIfLoggedIn=0) OR 
       (RequireLogin=0 AND HideIfLoggedIn=1) OR 
       (RequireLogin=0 AND HideIfLoggedIn=0))

Answer 2

If I have the algebra correct:

alter table dbo.Example
add constraint RequireAdmin_RequireLogin_ck
check ( not ( RequireAdmin = 'true' and RequireLogin = 'false' ) ) ;
alter table dbo.Example
add constraint HideIfLoggedIn_RequireLogin_ck
check ( not ( HideIfLoggedIn = 'true' and RequireLogin = 'true' ) ) ;

Note that these check constraints are defined at the table level not the column level, as they must because they reference multiple columns. RThomas' answer is not valid Transact-SQL for this reason!