How to Efficiently Find Content Matching Multiple Tags in SQL?

SQL Query: Efficient Intersections for Multiple Tags

When working with a table of tags assigned to content, it's common to need to identify content that matches specific tag combinations. A naive approach using nested subqueries can quickly become unwieldy for multiple tags.

To achieve this intersection efficiently, we can leverage the following SQL query:

SELECT contentID
FROM tags
WHERE tagID in (334, 338, 342)
GROUP BY contentID
HAVING COUNT(DISTINCT tagID) = 3;

Explanation:

• The WHERE clause selects rows where the tagID matches any of the provided tags (e.g., 334, 338, 342).
• The GROUP BY clause groups the results by contentID, ensuring each content piece is only represented once.
• The HAVING clause filters the results to include content pieces that have all the specified tags (in the example, COUNT(DISTINCT tagID) = 3).

By replacing the specific tag IDs with a parameterized list and adjusting the count in the HAVING clause, this query becomes generalizable for any number of tags:

SELECT contentID
FROM tags
WHERE tagID in (...) --taglist
GROUP BY contentID
HAVING COUNT(DISTINCT tagID) = ... --tagcount;

This optimized approach provides a robust solution for efficiently finding the intersection of multiple tags in a table.

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