UPDATE users SET reception_connu = NULL WHERE reception_connu = 0 OR reception_connu NOT IN (SELECT id FROM users);
UPDATE users SET reception_inconnu = NULL WHERE reception_inconnu = 0 OR reception_inconnu NOT IN (SELECT id FROM users);
ALTER TABLE users ADD FOREIGN KEY (reception_connu) REFERENCES users(id) ON DELETE SET NULL;
ALTER TABLE users ADD FOREIGN KEY (reception_inconnu) REFERENCES users(id) ON DELETE SET NULL;