Question

I am currently learning how to program in SQL. I have a table1 with firstname, lastname and roomname.
And I have another table2 that has all the roomnames.

I was wondering how to select firstname and lastname from those people who have been in ALL of the rooms. Do I need a GROUP BY?

Answer 1

For TransactSQL, if you know that every row in your first table is distinct, then this would work:

SELECT
   firstname,
   lastname
FROM
   table1
GROUP BY
   firstname,
   lastname
HAVING
   COUNT(DISTINCT RoomName) = (SELECT COUNT(*) FROM table2)

This should be pretty efficient, but not too flexible, in case you wanted to ignore certain rooms.