Trigger to delete rows from related tables before deleting rows from actual table

问题

I have the following tables:

CREATE TABLE QUESTION(
id varchar(10) NOT NULL PRIMARY KEY,
que_type numeric(1));

CREATE TABLE ESSAY(
que_id varchar(10) NOT NULL PRIMARY KEY,
ans varchar(2000),
FOREIGN KEY (que_id) REFERENCES QUESTION (id));

CREATE TABLE TFFB(
que_id varchar(10) NOT NULL PRIMARY KEY,
ans varchar(50),
FOREIGN KEY (que_id) REFERENCES QUESTION (id));

CREATE TABLE MCQ(
que_id varchar(10) NOT NULL PRIMARY KEY,
ans varchar(200),
FOREIGN KEY (que_id) REFERENCES QUESTION (id));

and try to create trigger so that when I delete from the main table, it will delete related rows from other tables:

CREATE OR REPLACE FUNCTION delete_question()
RETURNS trigger AS $delete_question$
DECLARE
    BEGIN
    IF ( (OLD).que_type = '1' ) THEN
       IF EXISTS (SELECT 1 FROM mcq WHERE person_id = (OLD).id) THEN
          DELETE FROM mcq WHERE que_id='(OLD).id';
       END IF;
    ELSIF ( (OLD).que_type = '2' OR OLD.que_type = '3' ) THEN
       IF EXISTS (SELECT 1 FROM tffb WHERE person_id = (OLD).id) THEN
          DELETE FROM tffb WHERE que_id='(OLD).id';
       END IF;
    ELSIF ( (OLD).que_type = '4' ) THEN
       IF EXISTS (SELECT 1 FROM essay WHERE person_id = (OLD).id) THEN
          DELETE FROM essay WHERE que_id='(OLD).id';
       END IF;
    END IF;
    RETURN NULL;
    END;
    $delete_question$ LANGUAGE plpgsql;

    CREATE TRIGGER delete_question
    BEFORE DELETE ON question
        FOR EACH ROW EXECUTE PROCEDURE delete_question();

When I delete data from question, the row disappear for a while. But when I refresh, it still there.
I tried to put RETURN OLD; but it failed because of the constrain relation. What is wrong with this?

回答1:

Some more advice on your trigger function:

CREATE OR REPLACE FUNCTION delete_question()
  RETURNS trigger AS
$func$
BEGIN

CASE OLD.que_type
WHEN 1 THEN
    DELETE FROM mcq   WHERE que_id=OLD.id;
WHEN 2, 3 THEN
    DELETE FROM tffb  WHERE que_id=OLD.id;
WHEN 4 THEN
    DELETE FROM essay WHERE que_id=OLD.id;
-- ELSE
--      Do something?
END CASE;

RETURN OLD;

END
$func$ LANGUAGE plpgsql;

Major points

Your check for existence with a SELECT statement doubles the cost. Just run the DELETE, if no matching row is found, nothing is deleted.
Use a CASE statement here. Shorter, faster. Note that plpgsql CASE is slightly different from SQL CASE statement. For instance, you can list several cases at once.
You don't need the DECLARE keyword, unless you actually declare variables.

Alternative design

You could avoid the problem altogether by cascading deletes via foreign key, as @a_horse mentioned in the comment. My schema layout would look like this:

CREATE TABLE question (
   question_id serial NOT NULL PRIMARY KEY
  ,que_type    int   -- this may be redundant as well
);

CREATE TABLE essay (
   que_id int NOT NULL PRIMARY KEY
              REFERNECES question(question_id) ON UPDATE CASCADE
                                               ON DELETE CASCADE
  ,ans    text
);

...

About serial:
Auto increment SQL function

回答2:

i then try solve it again.... IT WORKS PERFECTLY. YEAY.

CREATE OR REPLACE FUNCTION delete_question()
RETURNS trigger AS $delete_question$
DECLARE
BEGIN
IF ( OLD.que_type=1 ) THEN
   IF EXISTS (SELECT 1 FROM mcq WHERE que_id=OLD.id) THEN
      DELETE FROM mcq WHERE que_id=OLD.id;
   END IF;
ELSIF ( OLD.que_type=2 OR OLD.que_type=3) THEN
   IF EXISTS (SELECT 1 FROM tffb WHERE que_id=OLD.id) THEN
      DELETE FROM tffb WHERE que_id=OLD.id;
   END IF;
ELSIF ( OLD.que_type=4 ) THEN
   IF EXISTS (SELECT 1 FROM essay WHERE que_id=OLD.id) THEN
      DELETE FROM essay WHERE que_id=OLD.id;
   END IF;
END IF;
RETURN OLD;
END;
$delete_question$ LANGUAGE plpgsql;

CREATE TRIGGER delete_question
BEFORE DELETE ON question
    FOR EACH ROW EXECUTE PROCEDURE delete_question();

there is no ' ' on OLD.id or even (OLD).id and use RETURN OLD;

来源：https://stackoverflow.com/questions/20035105/trigger-to-delete-rows-from-related-tables-before-deleting-rows-from-actual-tabl

标签

sql

postgresql

triggers

plpgsql

sql-delete