PostgreSQL Check Constraint to Only Allow One of Two Booleans to be True

Question

Given:

CREATE TABLE my_table
(
    my_table_id serial NOT NULL PRIMARY KEY,
    flag_one boolean NOT NULL,
    flag_two boolean NOT NULL
);

Is there a way to create a check constraint to ensure that flag_one and flag_two cannot both be true? Both are allowed to be false and one or the other may be true.

Answer 1

CREATE TABLE my_table
(
    my_table_id serial NOT NULL PRIMARY KEY,
    flag_one boolean NOT NULL,
    flag_two boolean NOT NULL,

    check ( not (flag_one is true and flag_two is true) )
);