Why can't I reference a primary key? - mysql

Question

The first two tables successfully pass, but as soon as the mysql checker reach the third table, it informs me that there is a problem with the reference key.
Here is my code:
CREATE TABLE USER(
Username VARCHAR(20) NOT NULL,
Email VARCHAR(50) NOT NULL,
Picture VARCHAR(30),
Points INT(5) DEFAULT 1 NOT NULL,
Password VARCHAR(50) NOT NULL,
Firstname VARCHAR(25) NOT NULL,
Lastname VARCHAR(25) NOT NULL,
PRIMARY KEY(Username)
);
CREATE TABLE THREAD (
ThreadID INT(9) NOT NULL AUTO_INCREMENT,
Title VARCHAR(255) NOT NULL,
PostID int(9) NOT NULL,
PRIMARY KEY (ThreadID)
);
CREATE TABLE POST(
ThreadID INT(9) NOT NULL,
PostID INT(9) NOT NULL AUTO_INCREMENT,
Content TEXT NOT NULL,
NumberOfLines INT(5) NOT NULL,
PRIMARY KEY(ThreadID, PostID),
FOREIGN KEY ThreadID(ThreadID) REFERENCES THREAD(ThreadID)
);
The error I am getting is the following:
#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '
PRIMARY KEY(ThreadID, PostID),
FOREIGN KEY fk_ThreadID(ThreadID) REFE' at line 6
I tried creating the PRIMARY KEY(ThreadID, PostID) as separate entities, but this doesn't seem to work.
Thank you.

Answer 1

Adding to #Schwern, its giving error due to wrong usage of primary key. Below create table will work.
create table a (
id bigint(20) NOT NULL AUTO_INCREMENT,
col2 varchar(10), primary key (id, col2)
) engine=innodb;
So auto-increment column needs to be first one in the primary key.
We have extensively used this kind of primary key to partition the table on col2 - to range partition on a column, the column need to be part of primary key.
Also to mention, you can achieve additional unique key at the cost of an extra index

Answer 2

MySQL isn't being terribly informative there. SQLFiddle gives it straight.
Incorrect table definition; there can be only one auto column and it must be defined as a key
You can't have a multi-column key where one of them is AUTO_INCREMENT The AUTO_INCREMENT column must be first in a multi-column key in an InnoDB table. Either remove AUTO_INCREMENT from PostID, or make PostID alone the PRIMARY KEY and protect a post from appearing in the same thread twice with UNIQUE(ThreadID, PostID).
CREATE TABLE POST(
PostID INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
ThreadID INT NOT NULL REFERENCES Thread(ThreadID),
Content TEXT NOT NULL,
NumberOfLines INT NOT NULL,
UNIQUE(ThreadID, PostID)
);
UPDATE: The best answer is #georgecj11's to put PostID first in the key as it only uses a single index.