SQL - error code 1005 with error number 121 - mysql

Question

I'm running the following MySQL query (trimmed down), generated automatically by MySQL Workbench and I get the following error:
Error Code: 1005
Can't create table 'regula.reservation' (errno: 121)
I'm not very proficient with databases and this error is not very informative.
What is the problem here?
-- -----------------------------------------------------
-- Table `regula`.`Users`
-- -----------------------------------------------------
DROP TABLE IF EXISTS `regula`.`Users` ;
CREATE TABLE IF NOT EXISTS `regula`.`Users` (
`idUsers` INT UNSIGNED NOT NULL AUTO_INCREMENT ,
`name` TEXT NOT NULL ,
`type` TEXT NOT NULL ,
`pwd` TEXT NOT NULL ,
PRIMARY KEY (`idUsers`) ,
UNIQUE INDEX `idUsers_UNIQUE` (`idUsers` ASC) )
ENGINE = InnoDB;
-- -----------------------------------------------------
-- Table `regula`.`Projects`
-- -----------------------------------------------------
DROP TABLE IF EXISTS `regula`.`Projects` ;
CREATE TABLE IF NOT EXISTS `regula`.`Projects` (
`idProjects` INT UNSIGNED NOT NULL AUTO_INCREMENT ,
`ownerId` INT UNSIGNED NOT NULL ,
`name` TEXT NOT NULL ,
`date` DATE NOT NULL ,
`time` TIME NOT NULL ,
`place` TEXT NOT NULL ,
`itemType` INT NOT NULL ,
PRIMARY KEY (`idProjects`) ,
UNIQUE INDEX `idProjects_UNIQUE` (`idProjects` ASC) ,
INDEX `ownerId` (`ownerId` ASC) ,
CONSTRAINT `ownerId`
FOREIGN KEY (`ownerId` )
REFERENCES `regula`.`Users` (`idUsers` )
ON DELETE NO ACTION
ON UPDATE NO ACTION)
ENGINE = InnoDB;
-- -----------------------------------------------------
-- Table `regula`.`ItemTypes`
-- -----------------------------------------------------
DROP TABLE IF EXISTS `regula`.`ItemTypes` ;
CREATE TABLE IF NOT EXISTS `regula`.`ItemTypes` (
`idItemTypes` INT UNSIGNED NOT NULL ,
`prjId` INT UNSIGNED NOT NULL ,
`parentId` INT UNSIGNED NULL DEFAULT NULL ,
`name` TEXT NOT NULL ,
PRIMARY KEY (`idItemTypes`) ,
INDEX `prjId` (`prjId` ASC) ,
INDEX `parentId` (`parentId` ASC) ,
CONSTRAINT `prjId`
FOREIGN KEY (`prjId` )
REFERENCES `regula`.`Projects` (`idProjects` )
ON DELETE NO ACTION
ON UPDATE NO ACTION,
CONSTRAINT `parentId`
FOREIGN KEY (`parentId` )
REFERENCES `regula`.`ItemTypes` (`idItemTypes` )
ON DELETE NO ACTION
ON UPDATE NO ACTION)
ENGINE = InnoDB;
-- -----------------------------------------------------
-- Table `regula`.`Reservation`
-- -----------------------------------------------------
DROP TABLE IF EXISTS `regula`.`Reservation` ;
CREATE TABLE IF NOT EXISTS `regula`.`Reservation` (
`idReservation` INT UNSIGNED NOT NULL AUTO_INCREMENT ,
`prjId` INT UNSIGNED NOT NULL ,
`itemTypeId` INT UNSIGNED NOT NULL ,
`userId` INT UNSIGNED NOT NULL ,
PRIMARY KEY (`idReservation`) ,
INDEX `prjId` (`prjId` ASC) ,
INDEX `itemTypeId` (`itemTypeId` ASC) ,
INDEX `userId` (`userId` ASC) ,
CONSTRAINT `prjId`
FOREIGN KEY (`prjId` )
REFERENCES `regula`.`Projects` (`idProjects` )
ON DELETE NO ACTION
ON UPDATE NO ACTION,
CONSTRAINT `itemTypeId`
FOREIGN KEY (`itemTypeId` )
REFERENCES `regula`.`ItemTypes` (`idItemTypes` )
ON DELETE NO ACTION
ON UPDATE NO ACTION,
CONSTRAINT `userId`
FOREIGN KEY (`userId` )
REFERENCES `regula`.`Users` (`idUsers` )
ON DELETE NO ACTION
ON UPDATE NO ACTION)
ENGINE = InnoDB;

Answer 1

Error 121 means that there is a foreign key constraint error. Since you're using InnoDB, you can use SHOW ENGINE INNODB STATUS after running the failed query to get an explanation in the LATEST FOREIGN KEY ERROR section. Having run your SQL myself, I get this:
------------------------
LATEST FOREIGN KEY ERROR
------------------------
101210 14:55:50 Error in foreign key constraint creation for table `regula`.`Reservation`.
A foreign key constraint of name `regula`.`prjId`
already exists. (Note that internally InnoDB adds 'databasename'
in front of the user-defined constraint name.)
Note that InnoDB's FOREIGN KEY system tables store
constraint names as case-insensitive, with the
MySQL standard latin1_swedish_ci collation. If you
create tables or databases whose names differ only in
the character case, then collisions in constraint
names can occur. Workaround: name your constraints
explicitly with unique names.
Basically, you need to give your prjId constraint name a unique name in the last table. Constraint/foreign key names are global to a database, so they cannot be reused in different tables. Just change the last
CONSTRAINT `prjId`
to
CONSTRAINT `prjId2`

Answer 2

The error code 121 comes when you try to map the foreign key.
Basically it comes when your foreign key name already exists in the database.
For example:
ALTER TABLE `photokiosk`.`kiosk_event`
ADD CONSTRAINT `event_booking_id`
FOREIGN KEY `event_booking_id` (`event_booking_id`)
REFERENCES `event_booking` (`event_booking_id`)
If foreign key with the name event_booking_id is already mapped with the other table.
To get rid of this issue, please change the foreign key name, not the column name.

Answer 3

You will get this error message if you try to use a constraint name which already exists in the table.
Here 'prjId' already exists in table regula.ItemTypes. So, you can't use the same constraint name on table 'regula.reservation' again.