MySQL 8.0

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CREATE TABLE tableA (id VARCHAR(255));
CREATE TABLE tableB (id VARCHAR(255));
CREATE TABLE tableC (id VARCHAR(255));
CREATE TABLE tableABC (id VARCHAR(255) UNIQUE);
CREATE TRIGGER tr_bi_a BEFORE INSERT ON tableA 
FOR EACH ROW INSERT INTO tableABC VALUES (NEW.id);
CREATE TRIGGER tr_bi_b BEFORE INSERT ON tableB 
FOR EACH ROW INSERT INTO tableABC VALUES (NEW.id);
CREATE TRIGGER tr_bi_c BEFORE INSERT ON tableC 
FOR EACH ROW INSERT INTO tableABC VALUES (NEW.id);
INSERT INTO tableA VALUES ('keyA');    -- success
INSERT INTO tableB VALUES ('keyB');    -- success
INSERT INTO tableC VALUES ('keyC');    -- success
INSERT INTO tableA VALUES ('keyAA');   -- success
INSERT INTO tableB VALUES ('keyBB');   -- success
INSERT INTO tableC VALUES ('keyC');    -- already present in tableC

Duplicate entry 'keyC' for key 'tableABC.id'


INSERT INTO tableA VALUES ('keyC');    -- already present in tableC

Duplicate entry 'keyC' for key 'tableABC.id'


INSERT INTO tableB VALUES ('keyA');    -- already present in tableA

Duplicate entry 'keyA' for key 'tableABC.id'


INSERT INTO tableC VALUES ('keyABC');  -- success