Adding an one-out-of-two not null constraint in postgresql

Question

If I have a table in Postgresql:

create table Education ( 
    id                  integer references Profiles(id),
    finished            YearValue not null,
    started             YearValue,
    qualification       text,
    schoolName          text,
    studiedAt           integer references Organizations(id),
    primary key (id)
);

I need to make a constraint so that either schoolName or studiedAt needs to not be null (one of them has to have information in it).

How do I do this?

Answer 1

You can use a check constraint e.g.

constraint chk_education check (schoolName is not null or studiedAt is not null)

From the manual:

A check constraint is the most generic constraint type. It allows you to specify that the value in a certain column must satisfy a Boolean (truth-value) expression.

Edit: Alternative to comply with Pithyless' interpretation:

constraint chk_education check ((schoolName is not null and studiedAt is null) or (schoolName is null and studiedAt is not null))

Answer 2

You can also use a trigger on update and insert to check that a rule is followed before allowing the data into the table. You would normally use this type of approach when the check constraint needs more complicated logic.