DELIMITER $$
USE `database_name`$$
DROP TRIGGER /*!50032 IF EXISTS */  beforinsertblock $$
CREATE
/*!50017 DEFINER = 'root'@'localhost' */
TRIGGER beforinsertblock BEFORE INSERT ON `stack_over_t` 
FOR EACH ROW BEGIN
DECLARE COUNT INT(11);
select count(*) INTO COUNT from stack_over_t WHERE category='1';
  IF(COUNT>2) THEN 
     SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'STATMENT';
    END IF ;
END;
$$
DELIMITER ;

您可以阻止插入查询

使用insert ignore命令而不是insert命令。例子：
mysql>insert ignore into person_tbl（姓，名）->值（'jay'，'thomas'）；查询正常，1行受影响（0.00秒）
mysql>insert ignore into person_tbl（姓，名）->值（'jay'，'thomas'）；查询正常，0行受影响（0.00秒）

如果您使用的是trigger，则可以设置客户端和服务器端验证。

DELIMITER $$
CREATE TRIGGER example_before_insert_allow_only_one_active
     BEFORE INSERT ON stack_over_t FOR EACH ROW
     BEGIN
          IF (SELECT COUNT(id) FROM stack_over_t 
               WHERE id=NEW.id AND data=NEW.data) > 0
          THEN
               SIGNAL SQLSTATE '45000'
                    SET MESSAGE_TEXT = 'Cannot add or update row: only one active row allowed per type';
          END IF;
     END;
$$
CREATE TRIGGER example_before_update_allow_only_one_active
     BEFORE UPDATE ON stack_over_t  FOR EACH ROW
     BEGIN
          IF (SELECT COUNT(id) FROM stack_over_t 
               WHERE id=NEW.id AND data=NEW.data) > 0
          THEN
               SIGNAL SQLSTATE '45000'
                    SET MESSAGE_TEXT = 'Cannot add or update row: only one active row allowed per type';
          END IF;
     END;
$$