SQL Server : stored procedure or function - sql-server

Question

I have a table with 2 columns (A as bool and B as text), these columns can be:
both are null
if A is False, then B should be null
if A is True, then B should be not null
There are rules. I want to create a stored procedure or function to check these rules when row adding or updating (via trigger). What is better, stored procedure or function? If function, which type? In general, which variant is the best (return boolean or other way etc)?

Answer 1

I think you're after a CHECK Constraint.
Example:
ALTER TABLE Xxx
ADD CONSTRAINT chk_Xxx
CHECK ( (A IS NULL AND B IS NULL)
OR (A = 0 AND B IS NULL)
OR (A = 1 AND B IS NOT NULL)
) ;

Answer 2

I would use a CHECK CONSTRAINT wired up to a UDF.
Here is a silly example verifying that if you insert a Person, their age will be greater than 17.
if NOT exists (select * from sysobjects
where id = object_id('dbo.udfOlderThan17Check') and sysstat & 0xf = 0)
BEGIN
print 'Creating the stubbed version of dbo.udfOlderThan17Check'
EXEC ( 'CREATE FUNCTION dbo.udfOlderThan17Check ( #j as smallint ) RETURNS bit AS BEGIN RETURN 0 END')
END
GO
ALTER FUNCTION dbo.udfOlderThan17Check ( #Age smallint )
RETURNS bit AS
BEGIN
declare #exists int
select #exists = 0
if ( #Age IS NULL )
BEGIN
select #exists = 1 -- NULL VALUES SHOULD NOT BLOW UP THE CONSTRAINT CHECK
END
if ( #exists = 0 )
BEGIN
if #Age > 17
begin
select #exists = 1
end
END
return #exists
END
GO
IF EXISTS (SELECT * FROM dbo.sysobjects WHERE id = object_id(N'[dbo].[Person]') and OBJECTPROPERTY(id, N'IsUserTable') = 1)
BEGIN
DROP TABLE [dbo].[Person]
END
GO
CREATE TABLE [dbo].[Person]
(
PersonUUID [UNIQUEIDENTIFIER] NOT NULL DEFAULT NEWSEQUENTIALID()
, Age smallint not null
)
GO
ALTER TABLE dbo.Person ADD CONSTRAINT PK_Person
PRIMARY KEY NONCLUSTERED (PersonUUID)
GO
ALTER TABLE dbo.Person
ADD CONSTRAINT [CK_Person_AgeValue] CHECK ([dbo].[udfOlderThan17Check]( [Age] ) != 0)
GO
Here are some "tests":
INSERT INTO dbo.Person (Age) values (33)
INSERT INTO dbo.Person (Age) values (16)
INSERT INTO dbo.Person (Age) select 333 UNION select 58
INSERT INTO dbo.Person (Age) select 444 UNION select 4
select * from dbo.Person