How to Resolve 'Foreign Key Constraint Fails' Errors in MySQL?

Addressing the "Foreign Key Constraint Fails" Error in MySQL

When adding a foreign key to an existing table that references another, it's crucial to ensure that the referenced rows exist in the parent table. Otherwise, the "Cannot add or update a child row: a foreign key constraint fails" error can occur.

In the provided case, you are trying to add a foreign key (sourcecode_id) in the sourcecodes_tags table that references the sourcecodes table. However, based on your SHOW CREATE TABLE queries, it appears that there is no foreign key defined on the sourcecodes_tags table referencing the sourcecodes table.

The error message indicates that there is a foreign key violation, meaning that some sourcecode_id values in the sourcecodes_tags table do not exist in the sourcecodes table. To resolve this issue, you need to identify and remove those orphaned records.

Identifying Orphaned Records

To find the sourcecode_id values that do not exist in the sourcecodes table, you can use the following query:

SELECT DISTINCT sourcecode_id 
FROM sourcecodes_tags tags 
LEFT JOIN sourcecodes sc ON tags.sourcecode_id=sc.id 
WHERE sc.id IS NULL;

This query will return the sourcecode_id values that are not associated with any rows in the sourcecodes table.

Removing Orphaned Records

Once you have identified the orphaned records, you can delete them using a DELETE statement:

DELETE FROM sourcecodes_tags
WHERE sourcecode_id IN (
    SELECT DISTINCT sourcecode_id 
    FROM sourcecodes_tags tags 
    LEFT JOIN sourcecodes sc ON tags.sourcecode_id=sc.id 
    WHERE sc.id IS NULL
);

After removing the orphaned records, you should be able to add the foreign key constraint without any issues.

ALTER TABLE sourcecodes_tags 
ADD FOREIGN KEY (sourcecode_id) 
REFERENCES sourcecodes (id) 
ON DELETE CASCADE ON UPDATE CASCADE;

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