How to Efficiently Select Rows from One Table that Are Not Present in Another Table?

MySQL Selecting Rows Not Present in Another Table

To obtain all rows that exist in Table A but not in Table B while sharing common primary keys, a few methods are available, with varying performance implications.

Using a Subquery with NOT EXISTS:

Your initial approach with a subquery using NOT EXISTS is a viable option, but can be relatively slow, especially with larger datasets.

Using a Left Join:

As you discovered, a left join can perform faster. When joining Table A with Table B on the shared column, rows in Table A that do not match any rows in Table B will have NULL values in the join column. Filtering for these NULL values effectively isolates the desired rows.

Code Example:

<code class="sql">SELECT *
FROM A
LEFT JOIN B ON A.x = B.y
WHERE B.y IS NULL;</code>

Additional Tips:

• Ensure proper indexing on the join columns for both tables.
• Use a covering index on Table B to reduce the number of disk reads.
• Consider using a UNION query with a negative filter to explicitly exclude rows from Table B.

Overall Conclusion:

While the left join method typically outperforms the subquery approach, the optimal solution may vary depending on the specific dataset and schema. Experimentation and performance testing are recommended to determine the most efficient approach in each situation.

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