SQL：错误1005：无法创建表'obl2.itemsubjects'（错误号：121）

Question

I have the following tables:

CREATE  TABLE `OBL2`.`item` (
`itemID` INT NOT NULL AUTO_INCREMENT ,
`itemName` VARCHAR(45) NOT NULL ,
PRIMARY KEY (`itemID`) ,
INDEX `itemName` (`itemName` ASC) );

CREATE  TABLE `OBL2`.`subject` (
`subjectID` INT NOT NULL ,
`subjectName` VARCHAR(45) NOT NULL ,
PRIMARY KEY (`subjectID`) );

Now since the connection is many to many, each item can have many subject and each subject can be related to many items - I'd like to set a connection table. This is my code:

CREATE  TABLE `OBL2`.`itemsubjects` (
`itemID` INT NOT NULL ,
`subjectID` INT NOT NULL ,
PRIMARY KEY (`itemID`, `subjectID`) ,
INDEX `itemID_idx` (`itemID` ASC) ,
INDEX `subjectID_idx` (`subjectID` ASC) ,
CONSTRAINT `itemID`
FOREIGN KEY (`itemID` )
REFERENCES `OBL2`.`item` (`itemID` )
ON DELETE CASCADE
ON UPDATE CASCADE,
CONSTRAINT `subjectID`
FOREIGN KEY (`subjectID` )
REFERENCES `OBL2`.`subject` (`subjectID` )
ON DELETE CASCADE
ON UPDATE CASCADE);

but for some reason the code of the 3rd table is not being accepted. I get an error message:

ERROR 1005: Can't create table 'obl2.itemsubjects' (errno: 121)

I've read about the error on the internet and it says it's a known issue of MYSQL yet there are no solutions.

Any thoughts?

Answer 1

The MySQL docs say in FOREIGN KEY Constraints (emphasis mine):

If the CONSTRAINT symbol clause is given, the symbol value must be unique in the database . If the clause is not given, InnoDB creates the name automatically.

So, the reason that the itemsubject table creation failed, was that you had another (foreign key) constraint, named itemID , or one named subjectID in some other table of the database.

It's good to have a naming conevntion that is standard across the database. Just as you have ColumnName_idx for indices, you can use ReferencedTable_ReferencingTable_FK for foreign key constraints:

CREATE  TABLE OBL2.itemsubjects (
  itemID INT NOT NULL ,
  subjectID INT NOT NULL ,
  PRIMARY KEY 
    (itemID, subjectID) ,
  INDEX itemID_idx                           -- I like these 
    (itemID ASC) ,
  INDEX subjectID_idx                        -- two
    (subjectID ASC) ,
  CONSTRAINT item_itemsubject_FK             -- what I propose, here
    FOREIGN KEY (itemID)
    REFERENCES OBL2.item (itemID)
      ON DELETE CASCADE
      ON UPDATE CASCADE,
  CONSTRAINT subject_itemsubject_FK          -- and here 
    FOREIGN KEY (subjectID)
    REFERENCES OBL2.subject (subjectID)
      ON DELETE CASCADE
      ON UPDATE CASCADE
);