数据库触发器举例分析

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我们维护一张客户表，这张表的内容在周六和周日不允许修改。这就是客户表：

CREATE TABLE plch_customers(   custnum    INTEGER, custname   VARCHAR2 (100))

我需要创建一个触发器来阻止周末对这张表的任何插入、修改和删除。我写了这些代码：

CREATE OR REPLACE TRIGGER plch_customers_maint_trg   /*TEXT*/DECLAREBEGIN   IF TO_CHAR (SYSDATE, 'DY', 'NLS_DATE_LANGUAGE=ENGLISH')         IN ('SAT', 'SUN')   THEN      RAISE_APPLICATION_ERROR (-20000          , 'Customers maintained Monday-Friday only.');   END IF;END;/

哪些选项可用来取代文中的/*TEXT*/, 使得这个触发器不允许周末对plch_customers表执行任何DML操作，并且每个语句只触发一次？

(A)

BEFORE UPDATE ON plch_customers FOR EACH ROWBEFORE INSERT ON plch_customers FOR EACH ROWBEFORE DELETE ON plch_customers FOR EACH ROW

SQL> CREATE OR REPLACE TRIGGER plch_customers_maint_trgBEFORE UPDATE ON plch_customers FOR EACH ROWBEFORE INSERT ON plch_customers FOR EACH ROWBEFORE DELETE ON plch_customers FOR EACH ROWDECLAREBEGIN   IF TO_CHAR (SYSDATE, 'DY', 'NLS_DATE_LANGUAGE=ENGLISH')         IN ('SAT', 'SUN')   THEN      RAISE_APPLICATION_ERROR (-20000          , 'Customers maintained Monday-Friday only.');   END IF;END;ORA-04079: 无效的触发器说明SQL>

(B)

BEFORE UPDATE ON plch_customersBEFORE INSERT ON plch_customersBEFORE DELETE ON plch_customers

SQL> CREATE OR REPLACE TRIGGER plch_customers_maint_trgBEFORE UPDATE ON plch_customersBEFORE INSERT ON plch_customersBEFORE DELETE ON plch_customersDECLAREBEGIN   IF TO_CHAR (SYSDATE, 'DY', 'NLS_DATE_LANGUAGE=ENGLISH')         IN ('SAT', 'SUN')   THEN      RAISE_APPLICATION_ERROR (-20000          , 'Customers maintained Monday-Friday only.');   END IF;END;ORA-04079: 无效的触发器说明SQL>

(C)

BEFORE UPDATE, INSERT OR DELETE ON plch_customers

SQL> CREATE OR REPLACE TRIGGER plch_customers_maint_trgBEFORE UPDATE, INSERT OR DELETE ON plch_customersDECLAREBEGIN   IF TO_CHAR (SYSDATE, 'DY', 'NLS_DATE_LANGUAGE=ENGLISH')         IN ('SAT', 'SUN')   THEN      RAISE_APPLICATION_ERROR (-20000          , 'Customers maintained Monday-Friday only.');   END IF;END;ORA-00969: 缺失 ON 关键字SQL>

(D)

BEFORE UPDATE OR INSERT OR DELETE ON plch_customers FOR EACH ROW

SQL> Trigger createdSQL>

(E)

BEFORE UPDATE OR INSERT OR DELETE ON plch_customers

SQL> Trigger createdSQL>

答案E

D不是答案因为每行都会触发，不满足条件。

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