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    <p>Assuming you have a <code>users</code> table as well:</p> <pre><code>SELECT id FROM conversations AS c WHERE NOT EXISTS ( SELECT * FROM users AS u WHERE u.id IN (1, 2, 3, 4) AND NOT EXISTS ( SELECT * FROM conversations_users AS cu WHERE cu.user = u.id AND cu.conversation = c.id ) ) AND NOT EXISTS ( SELECT * FROM conversations_users AS co -- and only them WHERE co.conversation = c.id AND co.user NOT IN (1, 2, 3, 4) ) ; </code></pre> <p>If you don't have a <code>users</code> table or you don't like to use it (can'see why but anyway), you can replace this part:</p> <pre><code>WHERE NOT EXISTS ( SELECT * FROM users AS u WHERE u.id IN (1, 2, 3, 4) AND NOT EXISTS </code></pre> <p>with:</p> <pre><code>WHERE NOT EXISTS ( SELECT * FROM (SELECT 1 AS id UNION SELECT 2 UNION SELECT 3 UNION SELECT 4) AS u WHERE NOT EXISTS </code></pre> <hr> <p>The above query, while being general will not be very efficient in MySQL (blame the double nesting and the naive optimizer). The <code>GROUP BY / COUNT</code> way is probably a level more efficient - but please test with your data. You can also find many more ways (more than 10) to answer this kind of question, in this answer: <strong><a href="https://stackoverflow.com/questions/7364969/how-to-filter-sql-results-in-a-has-many-through-relation">How to filter SQL results in a has-many-through relation</a></strong> Some of them do not work in MySQL but many do. I'd expect queries 5 and 6 there to be quite efficient in MySQL (a level more efficient than the group by queries). </p> <p>Your case has a difference, you want exact relational division while that question/answers are about (simple) relational division, so you could write 5 like this:</p> <pre><code>SELECT id FROM conversations AS c WHERE EXISTS (SELECT * FROM conversations_users AS cu WHERE cu.conversation = c.id AND cu.user = 1) AND EXISTS (SELECT * FROM conversations_users AS cu WHERE cu.conversation = c.id AND cu.user = 2) AND EXISTS (SELECT * FROM conversations_users AS cu WHERE cu.conversation = c.id AND cu.user = 3) AND EXISTS (SELECT * FROM conversations_users AS cu WHERE cu.conversation = c.id AND cu.user = 4) AND NOT EXISTS (SELECT * FROM conversations_users AS cu WHERE cu.conversation = c.id AND cu.user NOT IN (1,2,3,4)) </code></pre>
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    1. POSELECT WHERE ON an advanced way
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    1. CO+1 relational division 1.0. Next step: find (and enumerate) equivalence groups of users that share the same set of conversations ;-)
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    2. COThanks a lot, but the `users` table is not needed for this. As the users ID's are in the `converstations_users` table.
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    3. COI didn't say it was needed. But if you do have a users table, the query is easier to write.
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