Question

现在我有一个包含student_id和section_id的courseenroll表的表，以及一个具有不同section_id和限制学生数的section表。和学生表有不同的student_id。

我是触发器的初学者。现在我需要触发约束。例如学生从网络用户界面jsp一个接一个地插入，如果学生人数大于本节中学生的数量，我应该向用户显示错误信息。 “要显示的错误消息应包括来自数据库服务器的错误文本，可能还附有您自己的消息，该消息明确标识了违反的约束。”

我尝试按照此链接中的示例SQL using trigger for constraint 修改如下：

CREATE TRIGGER enroll_out_of_limit on courseenroll
AFTER INSERT, UPDATE
AS
IF EXISTS (
           SELECT * 
             FROM section AS T
                  INNER JOIN (
                              SELECT section_id, COUNT(*) AS limitstu
                                FROM courseenroll
                               GROUP 
                                  BY section_id      
                             ) AS E
                                  ON T.section_id = E.section_id
                                     AND E.limitstu > T.limitstu
          )
BEGIN
RAISERROR ('A teachers''s student tally is too high to accept new students.', 16, 1);
ROLLBACK TRANSACTION;
RETURN 
END;

虽然它在“on”周围的代码开头给我错误，所以如何解决这个问题？感谢

节表有两个属性：section_id和limitstudent

学生表有一个属性：student_id

courseenroll有两个属性：student_id和section_id

只要课程表中某一部分的学生人数不超过相应的限制，本节的学生就可以了。

我按照教程进行操作我有以下触发器，但它在select语句

周围给出了错误

    CREATE OR REPLACE FUNCTION enroll_out_of_limit()
  RETURNS trigger AS
$BODY$
begin
    if (SELECT * 
        FROM section AS T
                INNER JOIN (
                              SELECT section_id, COUNT(*) AS limitstu
                              FROM courseenroll
                              GROUP BY section_id      
                             ) AS E
                                  ON T.section_id = E.section_id
                                     AND E.limitstu > T.limitstu ) THEN
                                     RAISE EXCEPTION 'this section enrolllist is full!';


        end if;

end;
$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;
ALTER FUNCTION enroll_out_of_limit()
  OWNER TO postgres;

如果我更改下面的sql，它可以创建触发器但是当我尝试插入时，它给出了“查询没有目标”的错误

    create or replace function enroll_out_of_limit() 
returns trigger as 
$BODY$
begin

        SELECT T.limitstu, E.currentsum
        FROM section AS T
        INNER JOIN (
                SELECT section_id, COUNT(*) AS currentsum
                FROM courseenroll
                GROUP BY section_id      
                ) AS E
        ON T.section_id = E.section_id ;
        if E.currentsum = T.limitstu 
        THEN
           RAISE EXCEPTION 'this section enrolllist is full!';
        end if; 

end;
$BODY$
    LANGUAGE 'plpgsql';

CREATE TRIGGER enroll_out_of_limit
before INSERT ON courseenroll for each ROW
EXECUTE PROCEDURE enroll_out_of_limit();

插入时的postgresql触发器不大于限制

0 个答案: