创建触发器阻止插入

Question

我正在尝试执行以下触发器：

create trigger t23 
on studies
after insert, update, delete 
as
begin
REFERENCING NEW ROW NewStudent
FOR EACH ROW
WHEN (30 <= (SELECT SUM(credits) FROM Studies)
DELETE FROM NewStudent N
WHERE N.spnr = NewStudent.spnr 
end

我正在尝试创建一个触发器，该触发器仅在学分 < 时插入学生。或 == 到“30”。 “Credits”是 int 类型。

我在尝试实现此触发器时遇到许多错误。我真的已经尝试了一切，但我别无选择。该领域的专家能否为我指出正确的方向？

Answer 1

CREATE TRIGGER MSDN 文档完全满足您的要求：

USE AdventureWorks2008R2;
GO
IF OBJECT_ID ('Purchasing.LowCredit','TR') IS NOT NULL
   DROP TRIGGER Purchasing.LowCredit;
GO
-- This trigger prevents a row from being inserted in the Purchasing.PurchaseOrderHeader table
-- when the credit rating of the specified vendor is set to 5 (below average).

CREATE TRIGGER Purchasing.LowCredit ON Purchasing.PurchaseOrderHeader
AFTER INSERT
AS
DECLARE @creditrating tinyint, @vendorid int;
IF EXISTS (SELECT *
           FROM Purchasing.PurchaseOrderHeader p 
           JOIN inserted AS i 
           ON p.PurchaseOrderID = i.PurchaseOrderID 
           JOIN Purchasing.Vendor AS v 
           ON v.BusinessEntityID = p.VendorID
           WHERE v.CreditRating = 5
          )
BEGIN
RAISERROR ('This vendor''s credit rating is too low to accept new purchase orders.', 16, 1);
ROLLBACK TRANSACTION;
RETURN 
END;

这里的关键是 ROLLBACK TRANSACTION，只需调整示例以满足您的需要即可。

编辑：这应该可以满足您的需求，但我尚未对其进行测试，因此您的里程可能会有所不同。

create trigger dbo.something after insert as
begin
    if exists ( select * from inserted where sum(credits) > 30 )
    begin
        rollback transaction
        raiserror ('some message', 16, 1)
    end
end

另一个编辑，基于一些假设（请注意，我即时编写了此脚本，因为我现在无法测试它）：

create table dbo.students
(
    student_id int not null,
    name varchar (50) not null
)

create table dbo.courses
(
    course_id int not null,
    name varchar (50) not null,
    required_credits int not null
)

create table dbo.results
(
    student_id int not null,
    course_id int not null,
    course_result int not null
)

create trigger dbo.check_student_results on dbo.results after insert as
(
    declare @check int

    select @check = count(*)
    from inserted as a
    join dbo.courses as b on b.course_id = a.course_id
    where b.required_credits > a.course.result

    if @check <> 0
    begin

        rollback transaction

        raiserror('The student did not pass the course.', 16, 1)

    end
)

这样，当您在 dbo.results 表中插入记录时，约束会检查如果学生已通过课程，并在适当的情况下取消插入。但是，最好在应用程序层检查这些东西。