按条件从一列到不同表的两列的外键

Question

I have tree tables MySQL:

articles table contains:

id int
title varchar(255)
...

news table contains:

id int
title varchar(255)
...

comments table contains:

id int
content text
type tinyint(1)  //this column holds 0 if this comment for news and holds 1 for article
fid  int         // this column holds id of article or news 

how can I make foreign key from comments table to articles and news. I mean how can implement this in MySQL query:

if type=0 then
 FOREIGN KEY (fid) REFERENCES news(id)

if type=1 then
 FOREIGN KEY (fid) REFERENCES articles(id)

Answer 1

To have proper PK-FK relationships I'd suggest to have a superset table (lets call it posts ). In that case your schema might look like

CREATE TABLE posts 
(
  id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  type TINYINT(1)
);

CREATE TABLE articles
(
  id INT NOT NULL,
  title VARCHAR (255),
  article_property VARCHAR(128),
  -- other article specific attributes
  CONSTRAINT FOREIGN KEY (id) REFERENCES posts (id)
);

CREATE TABLE news
(
  id INT NOT NULL,
  title VARCHAR (255),
  reporter VARCHAR(128),
  -- other news specific attributes
  CONSTRAINT FOREIGN KEY (id) REFERENCES posts (id)
);

CREATE TABLE comments
(
  id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
  post_id INT NOT NULL,
  content TEXT,
  CONSTRAINT FOREIGN KEY (post_id) REFERENCES posts (id)  
);

To populate id s when inserting new articles and news you can make use of triggers

DELIMITER $$
CREATE TRIGGER tg_article_insert 
BEFORE INSERT ON articles
FOR EACH ROW
BEGIN
  INSERT INTO posts (type) VALUES (1);
  SET NEW.id = LAST_INSERT_ID();
END$$

CREATE TRIGGER tg_news_insert 
BEFORE INSERT ON news
FOR EACH ROW
BEGIN
  INSERT INTO posts (type) VALUES (0);
  SET NEW.id = LAST_INSERT_ID();
END$$
DELIMITER ;

Here is SQLFiddle demo.

Answer 2

You can't. You can read about foreign key constraints here , and you will note that only one table is allowed.

One work-around is to have separate columns for the separate ids:

id int
content text
type tinyint(1)   //this column holds 0 if this comment for news and holds 1 for article
articleid  int
newsid int
. . . 
foreign key (articleid) references articles(id)
foreign key (newsid) references news(id)

In fact, you can dispense with the type and add in a constraint (implemented by a trigger) that only one id can be populated at any given time.