How to Delete Orphan Rows in MySQL Using JOINs, NOT EXISTS, or NOT IN?

Efficiently Removing Orphan Records in MySQL: Three Proven Methods

This guide demonstrates three effective techniques for deleting orphan rows in MySQL—database entries lacking corresponding records in a related table based on a shared ID.

Method 1: Leveraging LEFT JOIN and IS NULL

This approach utilizes a LEFT JOIN to combine data from two tables based on a specific ID column. Orphan rows, identified by a NULL value in the joined table's ID column, are then targeted for deletion.

<code class="language-sql">DELETE b FROM BLOB b
LEFT JOIN FILES f ON f.id = b.fileid
WHERE f.id IS NULL;</code>

Method 2: Employing NOT EXISTS

This method employs a subquery with NOT EXISTS to check for the presence of matching records in the related table. Rows lacking a match are flagged for deletion.

<code class="language-sql">DELETE FROM BLOB
WHERE NOT EXISTS (SELECT NULL
FROM FILES f
WHERE f.id = fileid);</code>

Method 3: Utilizing NOT IN

This straightforward approach uses NOT IN to compare IDs directly. Rows with IDs absent from the related table are identified and deleted.

<code class="language-sql">DELETE FROM BLOB
WHERE fileid NOT IN (SELECT f.id
FROM FILES f);</code>

Important Consideration:

Always execute DELETE statements within a transaction to ensure data integrity. This allows for rollback in case of errors or unintended consequences. Use START TRANSACTION before the DELETE and COMMIT (or ROLLBACK) afterward.

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