Question

我正在尝试编写一个比较旧值和新值的SQL触发器。如果这两个值不同，那么我需要显示一个错误，表示您无法更新名称。我的触发器的确切定义是

编写一个名为disallow_team_name_update的触发函数进行比较 OLD和NEW记录团队字段。如果他们是不同的提高表示不允许更改团队名称的例外情况。然后，将此触发器附加到名为tr disallow team的表中名称更新并指定在任何可能的更新之前触发表中的团队领域。

我正在使用的表格是：

  Table "table.group_standings"
    Column |         Type          | Modifiers
   --------+-----------------------+-----------
    team   | character varying(25) | not null
    wins   | smallint | not null   
    losses | smallint | not null
    draws  | smallint | not null
    points | smallint| not null
   Indexes:
     "group_standings_pkey" PRIMARY KEY, btree (team)
  Check constraints:
   "group_standings_draws_check" CHECK (draws >= 0)
   "group_standings_losses_check" CHECK (losses >= 0)
   "group_standings_points_check" CHECK (points >= 0)
   "group_standings_wins_check" CHECK (wins >= 0)

这是我的代码：

CREATE OR REPLACE FUNCTION disallow_team_name_update() RETURNS trigger AS $$
    BEGIN
            if(NEW.team <> OLD.team)
            /*tell the user to not change team names*/

    END;
$$ LANGUAGE plpgsql;


CREATE TRIGGER tr_disallow_team_name_update BEFORE INSERT OR UPDATE OF team ON group_standings 
FOR EACH ROW EXECUTE PROCEDURE disallow_team_name_update();

Answer 1

PostgreSQL可以使用raise exception来引发异常。

CREATE OR REPLACE FUNCTION disallow_team_name_update() 
RETURNS trigger AS 
$$
    BEGIN
        if(NEW.team <> OLD.team) then
            raise exception 'Invalid update.'
            using hint = 'Changes to team name are not allowed.';
        end if;

    END
$$ 
LANGUAGE plpgsql;

您肯定不希望禁止对 insert 上的团队名称进行更改。（无论如何，PostgreSQL都不允许你这样做。）

CREATE TRIGGER tr_disallow_team_name_update 
BEFORE UPDATE OF team ON group_standings 
FOR EACH ROW EXECUTE PROCEDURE disallow_team_name_update();

编写SQL触发器以比较旧值和新值

1 个答案: