来自同一表的MySql外键（两列与一个键相关）

Question

New question since I managed to find the answer.

The exercise wants us to create a comments table with reply levels. there is a post ('id') that has comments ('comment_id') and there is a comment reply field ('reply_to') that needs to point to an existing comment ('comment_id') or NULL in case it is a comment to the post itself. 'id' & 'comment_id' are the primary key 'user_id' is a foreign key for a different table i need a 2nd foreign key which is the reply field ('reply_to') and on delete of a comment the following comments should be deleted as well.

what I succeeded was setting the foreign key to 'comment_id' but then if there are many posts with the same comment number it causes chaos. deleting post #1 comment #1 will result in post #x comment #9 which is a reply to comment #5 (and not #1) to be deleted as well.

How should I define the key so that only the right comment tree will be eliminated? (description of the db below)

Thanks

Example: tbl

post_id-----comment_id-----reply_to   
   0            0             NULL (to post)
   0            1              0
   1            0              NULL (to post)
   1            1              0

wanted action:

Delete:       0            0             NULL (to post)

wanted result:

post_id-----comment_id-----reply_to   
   1            0              NULL (to post)
   1            1              0

===========================================================================

Fixed: needed to add indexing

The exercise wants us to create a comments table with reply levels. there is a post ('id') that has comments ('comment_id') and there is a comment reply field ('reply_to') that needs to point to an existing comment ('comment_id') or NULL in case it is a comment to the post itself. 'id' & 'comment_id' are the primary key 'user_id' is a foreign key for a different table i need a 2nd foreign key which is the reply field ('reply_to') and on delete of a comment the following comments should be deleted as well.

I am trying to add a foreign key from the same table but I keep getting this error:

#1005 - Can't create table db.#sql-148f_1027d34' (errno: 150)

Here is the definition of the table:

CREATE TABLE IF NOT EXISTS `PostComments` (
  `id` int(11) NOT NULL,
  `comment_id` int(11) NOT NULL,
  `user_id` int(11) NOT NULL,
  `content` varchar(512) COLLATE utf8_unicode_ci NOT NULL,
  `reply_to` int(11) DEFAULT NULL,
  `rating` int(11) NOT NULL DEFAULT '0',
  `report_counter` int(11) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`,`comment_id`),
  KEY `user_id` (`user_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;


ALTER TABLE `PostComments`
  ADD CONSTRAINT `PostComments_ibfk_1` FOREIGN KEY (`id`) REFERENCES `Posts` (`id`),
  ADD CONSTRAINT `PostComments_ibfk_2` FOREIGN KEY (`id`) REFERENCES `Posts` (`id`),
  ADD CONSTRAINT `PostComments_ibfk_3` FOREIGN KEY (`user_id`) REFERENCES `Users` (`id`);

this is what i am trying to do:

ALTER TABLE `PostComments`
ADD CONSTRAINT `fk_ReplyToT`
FOREIGN KEY (`reply_to`)
REFERENCES `PostComments` (`comment_id`)
ON DELETE CASCADE

After strolling in a few different threads I tried deleting all of the entries in this table but still no success. Also the fields are with the same type (INT)

what else am I possibly missing? Thanks in advance!

Answer 1

You are trying to reference two times the same column from the table postcomments with the same id in the table posts:

ALTER TABLE `PostComments`
  ADD CONSTRAINT `PostComments_ibfk_1` FOREIGN KEY (`id`) REFERENCES `Posts` (`id`),
  ADD CONSTRAINT `PostComments_ibfk_2` FOREIGN KEY (`id`) REFERENCES  `Posts` (`id`),
  ADD CONSTRAINT `PostComments_ibfk_3` FOREIGN KEY (`user_id`) REFERENCES `Users` (`id`);

That has no use.

The first ( id ) in the FOREIGN KEY statement (so FOREIGN KEY ( id ) ) is the column in the table you are applying the constraint on ( the postcomments table in this case).

So replace the id of your second constraint (of first, does not matter) with the column you actually want to place the constraint on.