Question

I have a nullable column that should be only assignable once. Once set, any attempt to overwrite the value should fail. Can this be accomplished with a CHECK constraint (i.e. is it possible to look at pre-update value of the column and compare with the new value) or can this only be accomplished with an UPDATE TRIGGER?

Thank you!

Answer 1

You can do this with an update trigger, with a query like this:

update t1 set
    field = d.field
from
    table t1
    inner join inserted i on
        t1.id = i.id
    inner join deleted d on
        t1.id = d.id
where
    i.field != d.field

A check constraint has no temporal recognition, that I’m aware of.