Postgres 13

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CREATE TABLE zoo (
  cage   INTEGER,
  animal TEXT,
  is_aggressive BOOLEAN,
  CONSTRAINT no_different_animals_in_same_cage 
	EXCLUDE USING gist (cage WITH =, animal WITH <>) WHERE (is_aggressive)
);
INSERT INTO zoo VALUES(1,'zebra',true);
INSERT INTO zoo VALUES(1,'cat',false);
INSERT INTO zoo VALUES(1,'bird',false);
INSERT INTO zoo VALUES(1, 'rabbit', false)
1 rows affected

1 rows affected

1 rows affected

1 rows affected

INSERT INTO zoo VALUES(1, 'lion', true)
ERROR:  conflicting key value violates exclusion constraint "no_different_animals_in_same_cage"
DETAIL:  Key (cage, animal)=(1, lion) conflicts with existing key (cage, animal)=(1, zebra).