mysql error #1452 appearing when trying to add a constraint

Posted by user1701484 on Stack Overflow See other posts from Stack Overflow or by user1701484
Published on 2012-09-30T09:31:18Z Indexed on 2012/09/30 9:37 UTC
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I am trying to alter a table so That I can add a foreign key constraint in mysql database:

ALTER TABLE  `Question` ADD CONSTRAINT `FK_question` FOREIGN KEY (`QuestionId`)
REFERENCES `Image_Question` (`QuestionId`) ON DELETE CASCADE ;

Problem is that it is giving me this error:

1452 - Cannot add or update a child row: a foreign key constraint fails (mobile_app.

'#sql-4517_15241'>, CONSTRAINT FK_question FOREIGN KEY (QuestionId) REFERENCES Image_Question (QuestionId) ON DELETE CASCADE)

What does this error actually mean and what are the possible solutions I might have to undertake in order to fix this?

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