Question

I want to perform some business validation of the data being inserted in a table and I need to check data in another table when doing so. The way to this seems to be using a BEFORE INSERT FOR EACH ROW TRIGGER in PostgreSQL and from the function it calls I can return NULL if the new data fails validation to prevent it being inserted.

Is there any way I can return an error message describing the validation error as well?

Answer 1

Use raise for exceptions:

39.8. Errors and Messages

Use the RAISE statement to report messages and raise errors.
[…]
EXCEPTION raises an error (which normally aborts the current transaction);

So you can say things like this in your trigger:

raise exception 'Say something useful about %', some_variable;

That will abort the trigger and (usually) the current transaction. The message that you raise should make its way back to the client application.