How to Efficiently Find Users with Multiple Tags Using SQL Joins?

SQL for Applying Conditions to Multiple Rows in a Join

Problem:

Determine users who possess both 'tag1' and 'tag2' tags while avoiding the effect of using 'IN,' which returns users with either tag.

Solution:

To select rows from table A based on two row conditions in table B, choose one of these strategies:

1. Testing Different Rows:

• EXISTS Clause: Verifies the existence of rows in table B that match the conditions.

SELECT * FROM users WHERE 
  EXISTS (SELECT * FROM tags WHERE user_id = users.id AND name ='tag1') AND
  EXISTS (SELECT * FROM tags WHERE user_id = users.id AND name ='tag2')

• Sub-queries: Matches based on rows in table B.

SELECT * FROM users WHERE 
  id IN (SELECT user_id FROM tags WHERE name ='tag1') AND
  id IN (SELECT user_id FROM tags WHERE name ='tag2') 

• JOINs: Aggregates rows by joining with table B multiple times.

SELECT u.* FROM users u 
     INNER JOIN tags t1 ON u.id = t1.user_id
     INNER JOIN tags t2 ON u.id = t2.user_id
  WHERE t1.name = 'tag1' AND t2.name = 'tag2'

2. Aggregating Rows:

• COUNTs: Suitable if table B protects against duplicate tags.

SELECT users.id, users.user_name 
   FROM users INNER JOIN tags ON users.id = tags.user_id
   WHERE tags.name IN ('tag1', 'tag2')
   GROUP BY users.id, users.user_name
   HAVING COUNT(*) = 2

• String Processing (not standard SQL): Concatenates tags into a string and checks for the presence of specific tags.

SELECT user.id, users.user_name, GROUP_CONCAT(tags.name) as all_tags
  FROM users INNER JOIN tags ON users.id = tags.user_id
  GROUP BY users.id, users.user_name
  HAVING FIND_IN_SET('tag1', all_tags) > 0 AND
         FIND_IN_SET('tag2', all_tags) > 0 

Recommendation:

For optimal scalability, consider using COUNTs with protected tags or inner aggregation if multiple tags can appear for a user.

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