How to Efficiently Find the Intersection of Multiple Row Conditions in SQL?

Solving the "AND" Intersection Over Multiple Rows in SQL

Querying a database to retrieve data based on multiple conditions is a common task. However, when the conditions need to be applied over multiple rows, the approach can become complex. This article presents a more efficient solution for such scenarios, eliminating the need for sub-querying.

Consider the "tags" table with columns "tagid" and "contentid." To find the "contentid" of every piece of content tagged with tagids 334, 338, and 342, the traditional approach involves nested sub-queries. However, this method becomes less efficient as the number of conditions increases.

The optimized approach avoids sub-queries, resulting in faster and more extensible query performance. Here's the revised query:

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

This query achieves the intersection by utilizing the "IN" operator with a list of tagids. The "GROUP BY" clause groups the results by "contentID," and the "HAVING" clause filters the results to include only content that has all the specified tagids.

In general, this pattern can be extended to find the intersection of any number of tags:

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

By replacing the "taglist" and "tagcount" placeholders with the desired values, this query can efficiently find the content that meets the intersection criteria.

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