Question

Suppose that I have a table my_table(id, x, y). I want to write a trigger to prevent updating the y col and setting it to a non-null value if x is already null. As SQL Server doesn’t have a before update trigger, how can this be done? Apparently we can use an instead of trigger for this purpose, but how can we check the old and current values and decide whether we should raise an error or let the update execute normally?

Example:

Let’s pretend we have this row in the DB:

1, null, null

Then this should fail (raise error)

update my_table set y = 'blah' where id = 1;

But this should succeed:

update my_table set y = null where id = 1;

I know the example isn’t very meaningful, but it is similar to what I am trying to achieve.

Answer 1

This should work, but I am not sure what other edge conditions you need to handle:

create table my_table (id int identity, x varchar(20), y varchar(20))
go 
CREATE TRIGGER tgNotNullYonMyTable
ON my_table
FOR UPDATE
AS 
IF UPDATE(y)
   BEGIN
       IF exists (
                  select 1 
                    from deleted d 
                         join inserted i on i.id = d.id 
                   where (d.x is null or i.x is null)
                         and i.y is null
                 ) 
          BEGIN
              RAISERROR ('Leave Y alone if x is null.', 16, 1)
              rollback tran
          END
   END
go

insert my_table values (null,null)

go 

update my_table set y = 'blah' where id = 1;
go 
update my_table set y = null where id = 1;