Update table to make primary key out of two foreign keys

Question

Have a table sample_tag which already exists with 1000's of entries.

It consists of two foreign keys, sample_id and tag_id .

However, the database is allowing duplicate sample_id/tag_id records to be created.

Without creating a new table, is there a SQL statement to update the sample_tag table such that the two foreign keys, together, function as its primary key?

Database is MySQL using phpMyAdmin

Answer 1

The main challenge is how you resolve the duplicates. If there are already duplicate keys, then you'll need to address these as a separate step (either deleting the duplicate, or merging any other columns as you see fit).

To check for duplicates, try:

SELECT sample_id, tag_id, COUNT( * )
FROM sample_tag
GROUP BY sample_id, tag_id;

If there are none, then you can go ahead and delete any existing primary key, and create a new one.