添加外键时出错

Question

I had a table named movies which had the fields id as primary key, and two varchars: title and genre .

I created a new table named genres with the int field id as primary key and desription varchar. I changed the field genre in my movies table so I could create a foreign key referencing a genre.

However, Mysql Workbench says there's an error when creating the foreign key.

Here's the statement:

ALTER TABLE `managemovies`.`movies` 
  ADD CONSTRAINT `genre_reference`
  FOREIGN KEY (`genre` )
  REFERENCES `managemovies`.`genres` (`id` )
  ON DELETE NO ACTION
  ON UPDATE NO ACTION
, ADD INDEX `genre_reference_idx` (`genero` ASC) ;

Error:

ERROR 1452: Cannot add or update a child row: a foreign key constraint fails  (`managemovies`.`#sql-3ba_2b`, CONSTRAINT `genre_reference` FOREIGN KEY (`genre`) REFERENCES `genres` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION)

SQL Statement: [... same statement than above ... ] ERROR: Error when running failback script. Details follow.

ERROR 1046: No database selected
SQL Statement:
CREATE TABLE `movies` [...]

[... the errors above repeated again ...]

Answer 1

clear your table contents and try adding foreign key.

if your table contain data which not matching the foreign key field value you will see this error ...

Answer 2

It looks like your table movies has data in genre column which is not present in genres.id column.

Your statement should work after removing the invalid data.

Hope it helps

Vishad