Postgres constraint ensuring one column of many is present?

Question

What are good ways to add a constraint to PostgreSQL to check that exactly one column (from a set of columns) contains a non-null value?

Update: It is likely th

Answer 1

As hinted by mu is too short:

alter table t
add constraint only_one_null check (
    (col1 is not null)::integer + (col2 is not null)::integer = 1
)