Question

I have two dependent tables as

CREATE TABLE posts
(
post_id int(11) unsigned NOT NULL AUTO_INCREMENT,
title varchar(255),
PRIMARY KEY(post_id)
) ENGINE=InnoDB

CREATE TABLE post_meta
(
post_id int(11) unsigned REFERENCES posts(post_id) ON DELETE CASCADE,
info varchar(255),
PRIMARY KEY(post_id)
) ENGINE=InnoDB

Question 1: After INSERTing into posts, post_meta does not accept value with error Duplicate entry XX for key 'PRIMARY. How should I modify the table structure?

Question 2: How can I set to create a corresponding row in post_meta upon INSERT INTO posts? I mean creating an empty row (only having id of the FK) in post_meta when creating a row in posts. In other words having the same number of rows in two columns without any INSERT into the second column.

Answer 1

Your current implementation looks as out of normalized form. Are you sure you need to keep the data separated to two different tables? Maybe

CREATE TABLE posts
(
post_id int(11) unsigned NOT NULL AUTO_INCREMENT,
title varchar(255),
info varchar(255),
PRIMARY KEY(post_id)
) ENGINE=InnoDB

will do?

Speculating: if you’re doing it that way because of security issues then MySQL supports Column-level privileges.

If normalizing your data is unacceptable for some reasons then you can just make post_id primary key in both tables (don’t make it foreign key!) and add INSERT and DELETE triggers on table posts which will insert or delete corresponding rows from post_meta.

UPDATE

You said in comment that you can have multiple records in post_meta table corresponding to single record in posts. That changes a lot:

1. You MUST NOT use primary key on post_id in table post_meta. Primary key MUST be unique in table scope.
2. You can set up your foreign key (the one you already tried to) – this will ensure that metadata is deleted automatically with posts.
3. If you do need PRIMARY KEY defined on post_meta then you shoud add a new (possibly auto_increment field) to post_meta table and use it as primary key. (Also, table can exist even without primary key but it’s against most DB guidelines)
4. If you need to create a meta record(s) automatically for each post then you can add an INSERT trigger on posts as I’ve already suggested. Another approach is using a stored procedure (and only stored procedure) for adding records to posts – and in this SP you can write some SQL to insert necessary records to post_meta.