编写 SQL 触发器来比较新旧值

Question

我正在尝试编写一个 SQL 触发器来比较新旧值。如果两个值不同，那么我需要显示一条错误，指出您无法更新名称。我的触发器的确切定义是

编写一个名为 disallow_team_name_update 的触发器函数，用于比较 OLD 和 NEW 记录团队字段。如果它们不同，则会引发异常，指出不允许更改团队名称。然后，将此触发器附加到名称为 tr disallow team name update 的表，并指定它在表中团队字段的任何潜在更新之前触发。

我用于解决此问题的表是：

  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 可以用来raise exception，嗯，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;

您肯定不想禁止在insert上更改团队名称。（无论如何，PostgreSQL 不会让你这样做。）

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