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

Since PostgreSQL 9.6 you have the num_nonnulls and num_nulls comparison functions that accept any number of VARIADIC arguments.

For example, this would make sure exactly one of the three columns is not null.

ALTER TABLE your_table
ADD CONSTRAINT chk_only_one_is_not_null CHECK (num_nonnulls(col1, col2, col3) = 1);