Question

我想在PostgreSQL中创建与SQL Server中相同的触发器。

我尝试将代码更改为可运行的功能，但出现了很多错误，但未指明错误之处，但以某种方式设法更改了可以运行的代码，但是调用触发器后却出现了错误：

旧的关系不存在

SQL Server触发器：

CREATE TRIGGER DeleteUser 
ON users
INSTEAD OF DELETE
AS
BEGIN
    IF EXISTS (SELECT * 
               FROM deleted d
               LEFT JOIN orders as o on o.user_id = d.id
               WHERE o.id IS NOT NULL)
    BEGIN
        ROLLBACK
        RAISERROR('Cannot delete this record: user already ordered something',16,1)
    END
    ELSE
    BEGIN
        DELETE FROM users
        WHERE EXISTS (SELECT * FROM deleted d WHERE d.id = users.id)

        PRINT('Deleted!')
    END
END
GO

适用于Postgres的代码：

功能：

CREATE FUNCTION delete_user()
RETURNS TRIGGER
AS $$
BEGIN
IF EXISTS (
    SELECT * 
    FROM old d
    LEFT JOIN orders as o on o.user_id = d.id
    where o.id is not null
    ) THEN
    RAISE EXCEPTION 'Cannot delete this record: user already ordered something';
ELSE
      DELETE FROM users
      WHERE EXISTS (Select * from deleted d where d.id = users.id);
      RAISE NOTICE 'Deleted!';
END IF;
END;
$$
LANGUAGE plpgsql;

触发：

CREATE TRIGGER deletion_of_user
    BEFORE DELETE
    ON public.users
    FOR EACH ROW
    EXECUTE PROCEDURE public.delete_user();

Answer 1

您无需在触发代码中删除。因为它是before触发器，所以仅返回old记录就足以进行删除。

您创建了行级别触发器（SQL Server没有此触发器），因此old记录仅是一行，您无法从中选择。只需在SELECT语句的WHERE子句中使用它即可。

CREATE FUNCTION delete_user()
RETURNS TRIGGER
AS $$
BEGIN
IF EXISTS (
    SELECT * 
    FROM orders as o 
    where o.user_id = old.id; --<< no need for a join
    ) THEN
    RAISE EXCEPTION 'Cannot delete this record: user already ordered something';
ELSE
    RAISE NOTICE 'Deleted!';
    return old; --<< tell Postgres to continue with the delete
END IF;
END;
$$
LANGUAGE plpgsql;

但是（错误地）为此使用触发器是错误的方法（在Postgres中以及在SQL Server中）。您应该从引用用户表的订单表中声明一个外键，这同样可以防止删除，而且效率可能更高。

如何更改SQL Server而不是删除PostgreSQL触发器

1 个答案: