How can I set a maximum number of rows in MySQL table?

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Try to make a restriction on adding a new record to a table. Raise an error when a new record is going to be added.

DELIMITER $$

CREATE TRIGGER trigger1
BEFORE INSERT
ON table1
FOR EACH ROW
BEGIN
  SELECT COUNT(*) INTO @cnt FROM table1;
  IF @cnt >= 25 THEN
    CALL sth(); -- raise an error
  END IF;
END
$$

DELIMITER ;

Note, that COUNT operation may be slow on big InnoDb tables.

On MySQL 5.5 you can use SIGNAL // RESIGNAL statement to raise an error.

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