Mysql Query Error #1005 150

Question

I have a query but its showing error #1005 and 150 in details. Some innodb

How to get it right?

CREATE TABLE `iars` ( `id` int(10) unsigned NOT NULL auto_increment,
`SessionId` int(10) unsigned NOT NULL,
`TeacherId` int(10) unsigned NOT NULL,
`courseid` int(4) unsigned NOT NULL,
`semester` int(4) unsigned NOT NULL,
`PaperId` int(4) unsigned NOT NULL,   
`groupid` int(4) unsigned NOT NULL,
`Type` varchar(4) default 1,
PRIMARY KEY  (`id`),
UNIQUE KEY `Constrint_Index` 
(`SessionId`,`TeacherId`,`courseid`,`semester`,`PaperId`,`groupid`),
KEY `FK_tid` (`TeacherId`),
KEY `FK_gid` (`GroupId`),
KEY `FK_pid` (`PaperId`),
CONSTRAINT `FK_gid` FOREIGN KEY (`GroupId`) REFERENCES `groups` (`id`) ON DELETE
CASCADE ON UPDATE CASCADE,
CONSTRAINT `FK_pid` FOREIGN KEY (`PaperId`) REFERENCES `papers` (`p_id`) ON DELETE
CASCADE ON UPDATE CASCADE,
CONSTRAINT `FK_ssessionid` FOREIGN KEY (`SessionId`) REFERENCES `sessions` (`id`) ON
DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `FK_tid` FOREIGN KEY (`TeacherId`) REFERENCES `teachers` (`id`) ON DELETE
CASCADE ON UPDATE CASCADE ) 

Answer 1

From the MySQL manual:

You cannot issue DROP TABLE for an InnoDB table that is referenced by a FOREIGN KEY constraint, unless you do SET foreign_key_checks = 0. When you drop a table, the constraints that were defined in its create statement are also dropped.

If you re-create a table that was dropped, it must have a definition that conforms to the foreign key constraints referencing it. It must have the right column names and types, and it must have indexes on the referenced keys, as stated earlier. If these are not satisfied, MySQL returns error number 1005 and refers to error 150 in the error message.

This means one of your other tables is probably referencing a column in iars which you are not re-creating. The solution, then, is to weed through all of your tables (via describe queries) and see where the reference is and either add the referenced column to your CREATE TABLE here, or remove the reference, as appropriate for your schema.

edit a very helpful gem from later on that referenced page:

SHOW ENGINE INNODB STATUS;

reveals loads of good info on the foreign-key error which just occurred. It should point you exactly where you need to look.