NOT EXISTS (
SELECT id
FROM teaches NATURAL JOIN course
GROUP BY id, time
HAVING COUNT(id) > 1);
创建触发器的SQL语句
CREATE TRIGGER example_trigger BEFORE INSERT ON teaches
FOR EACH ROW
EXECUTE PROCEDURE example_function();
example_function定义如下
CREATE FUNCTION example_function()
RETURNS TRIGGER
LANGUAGE plpgsql
AS $fun_trigger$
BEGIN
IF EXISTS (
SELECT id
FROM (SELECT id, course_name FROM teaches UNION (SELECT NEW.id, NEW.course_name)) AS T NATURAL JOIN course
GROUP BY id, time
HAVING COUNT(id) > 1)
THEN
RAISE EXCEPTION 'CONSTRAINT ERROR: A teacher cannot be in two classrooms at the same time';
END IF;
RETURN NEW;
END;
$fun_trigger$;
返回为TRIGGER(RETURNS TRIGGER)的存储过程中会自动创建一些变量
NEW:INSERT/UPDATE 操作新记录
OLD:DELETE/INSERT/UPDATE操作旧记录。更多的参数请参阅postgreSQL文档
course关系的数据:
SELECT course_name, room, time FROM course;
teaches关系的数据:
SELECT id, course_name FROM teaches;
插入一条有冲突的语句:
INSERT INTO teaches(id, course_name) VALUES('10003', 'Finance');
CREATE TRIGGER example_trigger BEFORE INSERT OR UPDATE ON teaches
FOR EACH ROW
EXECUTE PROCEDURE example_function();
2.如果你需要在一个表(TABLE)的指定列(COLUMN)更新时触发触发器,SQL为
CREATE TRIGGER example_trigger BEFORE UPDATE OF course_name ON teaches
FOR EACH ROW
EXECUTE PROCEDURE example_function();
3.如果你想用一个触发器(TRIGGER)作用在多个表上(TABLE),该怎么办呢?
CREATE TRIGGER example_trigger BEFORE INSERT ON teaches, course
FOR EACH ROW
EXECUTE PROCEDURE example_function();
不幸的是,这样的语法在postgreSQL中是不行的,你只能这样
CREATE TRIGGER example_trigger1 BEFORE INSERT ON teaches
FOR EACH ROW
EXECUTE PROCEDURE example_function();
CREATE TRIGGER example_trigger2 BEFORE INSERT ON course
FOR EACH ROW
EXECUTE PROCEDURE example_function();
参见点击查看详情
结束语:以上的SQL语句全部是在PostgreSQL(9.4.4)中运行通过,由于各种数据库的语法的差别,在其他数据库中并不能保证不会报错。由于本人水平所限,如有错误,欢迎批评指正。
参考文献:
1. 《数据库系统概念》第六版 5.3节 触发器
2. http://www.postgresql.org/docs/current/static/plpgsql-trigger.html
3. http://www.postgresql.org/docs/9.4/static/sql-createtrigger.html
4. http://www.tutorialspoint.com/postgresql/postgresql_triggers.htm
5. http://stackoverflow.com/questions/16102188/postgresql-insert-trigger-to-set-value
6. http://www.postgresql.org/message-id/17673.1129178350@sss.pgh.pa.us
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