Question

在我的数据库中，我有一个表Employee，它具有递归关联（员工可以是其他员工的老板）：

create table if not exists `employee` (

  `SSN` varchar(64) not null,
  `name` varchar(64) default null,
  `designation` varchar(128) not null,
  `MSSN` varchar(64) default null, 
  primary key (`ssn`),
  constraint `fk_manager_employee`  foreign key (`mssn`) references employee(ssn)

) engine=innodb default charset=latin1;

mysql> describe Employee;
+-------------+--------------+------+-----+---------+-------+
| Field       | Type         | Null | Key | Default | Extra |
+-------------+--------------+------+-----+---------+-------+
| SSN         | varchar(64)  | NO   | PRI | NULL    |       |
| name        | varchar(64)  | YES  |     | NULL    |       |
| designation | varchar(128) | NO   |     | NULL    |       |
| MSSN        | varchar(64)  | YES  | MUL | NULL    |       |
+-------------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

然后插入：

mysql> insert into Employee values
    -> ("1", "A", "OWNER",  NULL), 
    -> ("2", "B", "BOSS",   "1"),
    -> ("3", "C", "WORKER", "2"),
    -> ("4", "D", "BOSS",   "2"),
    -> ("5", "E", "WORKER", "4"),
    -> ("6", "F", "WORKER", "1"),
    -> ("7", "G", "WORKER", "4")
    -> ;
Query OK, 7 rows affected (0.02 sec)
Records: 7  Duplicates: 0  Warnings: 0

现在，我在表格中的行中有以下层次关系（所有者＆gt; boss＆gt; worker）：

     A
    / \
   B   F
  / \
 c   D
    / \
   G   E

以下是表格的Select语句：

mysql> SELECT * FROM Employee;
+-----+------+-------------+------+
| SSN | name | designation | MSSN |
+-----+------+-------------+------+
| 1   | A    | OWNER       | NULL |
| 2   | B    | BOSS        | 1    |  
| 3   | C    | WORKER      | 2    |  
| 4   | D    | BOSS        | 2    |  
| 5   | E    | WORKER      | 4    |   
| 6   | F    | WORKER      | 1    |  
| 7   | G    | WORKER      | 4    |  
+-----+------+-------------+------+
7 rows in set (0.00 sec)

现在，我想强加一个约束：If any employee (BOSS) deleted then new BOSS of workers under him become immediate BOSS of deleted employee (Old BOSS)。例如如果我删除D，则B成为G和E的BOSS。

为此，我还写了一个触发器，如下所示：

mysql>  DELIMITER $$
mysql>        CREATE
    ->        TRIGGER `Employee_before_delete` BEFORE DELETE
    ->          ON `Employee`
    ->          FOR EACH ROW BEGIN
    ->          UPDATE Employee
    ->          SET MSSN=old.MSSN
    ->          WHERE MSSN=old.MSSN; 
    ->        END$$
Query OK, 0 rows affected (0.07 sec)

mysql>        DELIMITER ;

但是当我执行一些删除时：

mysql> DELETE FROM Employee WHERE SSN='4';
ERROR 1442 (HY000): Can't update table 'Employee' in stored function/trigger
because it is already used by statement which invoked this stored 
function/trigger.

我learn here this trigger is not possible因为In MySQL triggers can't manipulate the table they are assigned to。

是否有一些其他可能的方法来执行此操作？是否可以使用Nested Query？有人可以建议我其他方法吗？一个建议就够了，但应该是有效的。

修改：
我得到了答案：可以使用stored procedure或two consecutive queries而不是触发器。 First和second。

我为此问题编写的解决方案如下，运作良好！：

正在为MYSQL version older then 5.5撰写的辅助信号函数。

DELIMITER //

CREATE PROCEDURE `my_signal`(in_errortext VARCHAR(255))
BEGIN
    SET @sql=CONCAT('UPDATE `', in_errortext, '` SET x=1');
    PREPARE my_signal_stmt FROM @sql;
    EXECUTE my_signal_stmt;
    DEALLOCATE PREPARE my_signal_stmt;
END//

从Employee表中删除员工的存储过程。

CREATE PROCEDURE delete_employee(IN dssn varchar(64))
BEGIN
    DECLARE empDesignation varchar(128);
    DECLARE empSsn         varchar(64);
    DECLARE empMssn        varchar(64);
     SELECT SSN, designation, MSSN  INTO empSsn, empDesignation, empMssn 
     FROM Employee 
     WHERE SSN = dssn;

   IF (empSsn IS NOT NULL) THEN
    CASE       
           WHEN empDesignation = 'OWNER' THEN 
               CALL my_signal('Error: OWNER can not deleted!');

           WHEN empDesignation = 'WORKER' THEN 
            DELETE FROM Employee WHERE SSN = empSsn;               

           WHEN empDesignation = 'BOSS' THEN 
               BEGIN 
                   UPDATE Employee
                   SET MSSN = empMssn
                   WHERE MSSN = empSsn;

                DELETE FROM Employee WHERE SSN = empSsn;                   

               END;
    END CASE;
   ELSE 
               CALL my_signal('Error: Not a valid row!');
   END IF;
END//

DELIMITER;

Answer 1

使用stored procedure：

UPDATE b
   SET b.mssn = a.mssn
  FROM EMPLOYEE a
  JOIN EMPLOYEE b ON b.mssn = a.ssn
 WHERE a.ssn = @deletedBoss

DELETE FROM employee WHERE ssn = @deletedBoss

使用存储过程，您只需删除所需的行，然后更新同一个表。这应该可以防止出现错误信息。

Answer 2

不进行硬删除，而是进行软删除。如果要删除行，请添加is_deleted列并将其设置为1。在表上创建after_update触发器。

DELIMITER $$
    CREATE
    TRIGGER `Employee_after_update` AFTER UPDATE
      ON `Employee`
      FOR EACH ROW BEGIN
      UPDATE Employee
      ...............
      WHERE NEW.is_deleted=1; 
      DELETE FROM Employee WHERE NEW.is_deleted=1
    END$$

我不确定您是否可以为after_ {insert，delete，update}更新同一个表。请检查

如何在MYSQL中删除相同的表？

2 个答案: