SQL query error - can't see why?

I'm currently reading through this Yii applica开发者_如何学Pythontion eBook: http://www.packtpub.com/agile-web-application-development-yii11-and-php5/book - and I'm having a problem inputting the tutorials DDL / SQL statements into PHPMyAdmin without it throwing up errors.

Would someone be kind enough to shed some light on why the following syntax is invalid? It might be something simple but I can't see it:

SQL statement:

ALTER TABLE tbl_issue
ADD CONSTRAINT FK_issue_project FOREIGN KEY (`project_id`)
REFERENCES tbl_project(`id`) ON DELETE CASCADE ON UPDATE RESTRICT;

ALTER TABLE tbl_issue
ADD CONSTRAINT `FK_issue_owner` FOREIGN KEY (`owner_id`)
REFERENCES tbl_user(`id`) ON DELETE CASCADE ON UPDATE RESTRICT;

ALTER TABLE tbl_issue
ADD CONSTRAINT `FK_issue_requester` FOREIGN KEY (`requester_id`)
REFERENCES tbl_user(`id`) ON DELETE CASCADE ON UPDATE RESTRICT;

ALTER TABLE tbl_project_user_assignment
ADD CONSTRAINT `FK_project_user` FOREIGN KEY (`project_id`)
REFERENCES tbl_project(`id`) ON DELETE CASCADE ON UPDATE RESTRICT;

ALTER TABLE tbl_project_user_assignment 
ADD CONSTRAINT `FK_user_project` FOREIGN KEY (`user_id`)
REFERENCES tbl_user(`id`) ON DELETE CASCADE ON UPDATE RESTRICT;

INSERT INTO tbl_user (`email`, `username`, `password`)
  VALUES
(`test1@notanaddress.com`,`Test_User_One`, MD5(`test1`)),
(`test2@notanaddress.com`,`Test_User_Two`, MD5(`test2`));

Error Message

Error
SQL query:

ALTER TABLE tbl_issue
ADD CONSTRAINT FK_issue_project FOREIGN KEY (  `project_id` ) 
REFERENCES tbl_project(  `id` ) ON DELETE CASCADE ON UPDATE RESTRICT ;


MySQL said:

#1005 - Can't create table 'trackstar_test.#sql-c78_127' (errno: 121) (<a    
href="server_engines.php?engine=InnoDB&amp;page=Status&amp;
token=252c0553975923580ca430b6e98c4243">Details...</a>)

Note:

• All the tables in the database are set to innodb as their storage engine.
• I've tried using different foreign key names for each FK, still get the same error.

Update:

• After finding no solution to the problem, I deleted by DB, uninstalled Xampp and then redid everything again. Seems to work now. Sorry to not be able to tell future readers exactly what the cause was, but it was most probably to do with my Database config or the information I added to it.

Actual Problem is :

AS you are Following the book, there are a few insert/ update statements are executed on

tbl_proect, 
tbl_issue

than you are trying to add Foreign Key Constraint. that checks the table data before applying. So, Here is the actual mistake, may be your tables contain a few records that violate the foreign key constraints. hence phpmyadmin doesnot allow you to alter table and generates error message.

Solution :

TRUNCATE TABLE `tbl_project`
TRUNCATE TABLE `tbl_issue`

Do Only one thing, clear all the tables . Empty tables. . And here's your problm resolved now phpmyadmin allows you to run commands.