DB2开发系列之四——触发器

1、触发器类型

1）BEFORE 触发器：在对表插入或更新之前执行该触发器，允许使用

CALL

和

SIGNAL

SQL 语句；

2）BEFORE DELETE 触发器：在删除操作之前执行该触发器；

3）AFTER 触发器：在更新、插入或删除操作之后执行。该触发器用于更新反映表间关系和一致性的其他表中的数据，还用于确保数据完整性。AFTER 触发器通常用于在特定情况下向用户生成报警；

4）INSTEAD OF 触发器：该触发器支持对不支持插入、更新和删除操作的视图执行这些操作；

2、创建触发器所需的权限

1）使用触发器的模式ID必须拥有以下权限之一：

      i)对定义 BEFORE 或 AFTER 触发器的表拥有 ALTER 权限；

      ii)对定义 INSTEAD OF 触发器的视图拥有 CONTROL 权限；

      iiI)对定义 INSTEAD OF 触发器的视图拥有所有权；

      iv)对定义触发器的表或视图的模式拥有 ALTERIN 权限；

      v)SYSADM 或 DBADM 授权；

2）以及以下任意一种权限：

      i)IMPLICIT_SCHEMA 数据库授权（如果触发器的隐式或显式模式名称不存在）；

      ii)对模式的 CREATEIN 权限（如果触发器的模式名称引用现有的模式）；

3）假定授权 ID 没有 SYSADM 和 DBADM 权限并且触发器存在，此 ID 必须拥有以下所有权限：

      i)对定义触发器的表拥有 SELECT 权限 — 用于转换变量和/或表；

      ii)对在触发的操作条件中引用的任意表或视图的 SELECT 权限；

      iii)调用触发的指定 SQL 语句所需的权限；

3、创建触发器语句

1）语法

.-NO CASCADE-.
>>-CREATE TRIGGER--trigger-name--+-+------------+--BEFORE-+----->
+-AFTER------------------+
'-INSTEAD OF-------------'

>--+-INSERT--------------------------+--ON--+-table-name-+------>
+-DELETE--------------------------+      '-view-name--'
'-UPDATE--+---------------------+-'
|     .-,-----------. |
|     V             | |
'-OF----column-name-+-'

>--+------------------------------------------------------------------+-->
|              .-------------------------------------------------. |
|              V  (1)    (2)          .-AS-.                     | |
'-REFERENCING------------------+-OLD--+----+--correlation-name-+-+-'
|      .-AS-.                   |
+-NEW--+----+--correlation-name-+
|            .-AS-.             |
+-OLD TABLE--+----+--identifier-+
|            .-AS-.             |
'-NEW TABLE--+----+--identifier-'

>--+-FOR EACH ROW--------------+--| triggered-action |--------->
|  (3)                      |
'--------FOR EACH STATEMENT-'

triggered-action

|--+-------------------------------------+---------------------->
|  (4)                                |
'--------WHEN--(--search-condition--)-'

2）触发器有三个主要组件：

      i)触发器事件；

      ii)触发器条件；

      iii)触发器操作；

3）示例：

      i)简单的 AFTER INSERT 触发器

      CREATE TRIGGER new_emp

      AFTER INSERT ON employee

      REFERENCING NEW AS n

      FOR EACH ROW

      INSERT INTO audit_emp VALUES (n.empno, 'Insert',0.0, current user, current timestamp)

      ii)简单的 AFTER DELETE 触发器

     CREATE TRIGGER purge_emp

     AFTER DELETE ON employee

     REFERENCING OLD AS o

     FOR EACH ROW

     INSERT INTO audit_emp VALUES (o.empno, 'Delete',0.0, current user, current timestamp)

      iii)简单的 AFTER UPDATE 触发器

     CREATE TRIGGER update_emp

     AFTER UPDATE OF salary ON employee

     REFERENCING OLD AS o NEW AS n

     FOR EACH ROW

     WHEN (n.salary <> o.salry)

      INSERT INTO audit_emp VALUES (o.empno,'Update',n.salary,current user, current timestamp)

      iv)简单的 BEFORE UPDATE 触发器

      CREATE TRIGGER update_bemp

      BEFORE UPDATE ON employee

      REFERENCING OLD AS o NEW AS n

      FOR EACH ROW

      WHEN (n.salary = 60000.00)

      SET n.salary = 75000.00)

4、触发器的高级用法

 1）INSTEAD OF 触发器

  --示例

    CREATE TABLE "DB2INST1"."EMPLOYEES"

    (

      "EMPNO"    CHAR(6) NOT NULL     ,

      "FIRSTNME" VARCHAR(12) NOT NULL ,

      "LASTNAME" VARCHAR(15) NOT NULL ,

      "PHONENO"  CHAR(4)              ,

      "SALARY"   DECIMAL(9,2)

      )

    CREATE VIEW employeev AS

    SELECT empno, firstnme, lastname, phoneno

    FROM employees

   CREATE TRIGGER new_emp1

    INSTEAD OF INSERT ON employeev

    REFERENCING NEW AS n

    FOR EACH ROW

    INSERT INTO employees VALUES (n.empno, n.firstnme, n.lastname, n.phoneno, 0)

 2）用触发器处理 XML 数据

  --示例

    CREATE TRIGGER new_order

    BEFORE INSERT ON purchaseorder

    REFERENCING NEW AS N

    FOR EACH ROW

      SET (n.porder) =  xmlvalidate(n.porder

      ACCORDING TOXMLSCHEMA URI 'http://posample.org/order.xsd')

 3）使用 SQL PL 语句扩展触发器主体

      i)语法

General Syntax Diagram for CREATE TRIGGER cont.
SQL-procedure-statement

|--+-CALL----------------------------------------------+--------|
+-Compound SQL (Dynamic)----------------------------+
+-FOR-----------------------------------------------+
+-+-----------------------------------+--fullselect-+
| |       .-,-----------------------. |             |
| |       V                         | |             |
| '-WITH----common-table-expression-+-'             |
+-GET DIAGNOSTICS-----------------------------------+
+-IF------------------------------------------------+
+-INSERT--------------------------------------------+
+-ITERATE-------------------------------------------+
+-LEAVE---------------------------------------------+
+-MERGE---------------------------------------------+
+-searched-delete-----------------------------------+
+-searched-update-----------------------------------+
+-SET Variable--------------------------------------+
+-SIGNAL--------------------------------------------+
'-WHILE---------------------------------------------'

      ii)从触发器中调用存储过程
      CREATE PROCEDURE write_audit( IN p_empno   CHAR(6),

                              IN p_txt     CHAR(6),

                              IN p_salary  DECIMAL(9,2),

                              IN p_user    CHAR(8),

                              IN p_curtime TIMESTAMP )

      BEGIN

      INSERT INTO audit_emp  VALUES ( p_empno, p_txt, p_salary, p_user, p_curtime )

      END

      CREATE TRIGGER new_emp

      AFTER INSERT ON employee

      REFERENCING NEW AS n

      FOR EACH ROW

      CALL write_audit( n.empno, 'Insert', 0.0, current user, current timestamp)

      iii)使用 SQL PL 的 BEFORE INSERT 触发器

      CREATE TRIGGER business_rules

      BEFORE INSERT ON empprojact

      REFERENCING NEW AS n

      FOR EACH ROW

      BEGIN ATOMIC

      -- Business Rule One (Project ending date Can't be NULL)

      IF (n.emendate IS NULL) THEN

      SET n.emendate = CURRENT date;

      END IF;

      -- Business Rule Two (Project ending date Can't end in last month of the year)

      IF (n.emendate BETWEEN '2009-12-01' AND '2009-12-31') THEN

     SIGNAL SQLSTATE '90000'

        SET MESSAGE_TEXT = 'Business Rule violation - 90000';

      END IF;

     END