Is it possible to make a unique value in the database?

[shizuka]

There is a field in the table whether a unique value can be added at the base level. For example, in value 1, that is, the unit with value 1 shall be 1 and the remaining values may be repeated.

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Find out what a trigger could be, a question, and how to stop the request from a trigger.

TRIGGER test.trigger1
 BEFORE INSERT
 ON test
 FOR EACH ROW
BEGIN
тут запрос
END

[Anderson]

You can really set up a trigger, two different triggers, one for the bet, one for the update.

DELIMITER //
CREATE TRIGGER check_insert_test BEFORE INSERT ON test
FOR EACH ROW BEGIN
  SET @block := (SELECT COUNT(*) FROM test WHERE test = 1);
  IF @block >= 1 THEN
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'INSERT canceled'; 
  END IF;
END//
CREATE TRIGGER check_test BEFORE UPDATE ON test
FOR EACH ROW BEGIN
  SET @block := (SELECT COUNT(*) FROM test WHERE test = 1 AND id <> NEW.id);
  IF @block >= 1 AND NEW.test = 1 THEN
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'UPDATE canceled'; 
  END IF;
END//
DELIMITER ;

Further implementation of requests can be interrupted by the use of SIGNAL SQLSTATE♪ The 45000 signal is specifically designed for user errors not intercepted. If the table contains a record of test = 1when attempting to insert a new record with the same value, or to update another record, trying to appoint test = 1♪, you'll get reports of a mistake ♪

INSERT INTO test VALUES (1, 1);
ERROR 1644 (45000): INSERT canceled
UPDATE test SET test = 1 WHERE id = 2;

ERROR 1644 (45000): UPDATE canceled