SQL: how to select a single id (“row”) that meets multiple criteria from a single column

Question

I have a very narrow table: user_id, ancestry.

The user_id column is self explanatory.

The ancestry column contains the country from where the user\'s ancest

Answer 1

First way: JOIN:

get people with multiple countries:

SELECT u1.user_id 
FROM users u1
JOIN users u2
on u1.user_id  = u2.user_id 
AND u1.ancestry <> u2.ancestry

Get people from 2 specific countries:

SELECT u1.user_id 
FROM users u1
JOIN users u2
on u1.user_id  = u2.user_id 
WHERE u1.ancestry = 'Germany'
AND u2.ancestry = 'France'

For 3 countries... join three times. To only get the result(s) once, distinct.

Second way: GROUP BY

This will get users which have 3 lines (having...count) and then you specify which lines are permitted. Note that if you don't have a UNIQUE KEY on (user_id, ancestry), a user with 'id, england' that appears 3 times will also match... so it depends on your table structure and/or data.

SELECT user_id 
FROM users u1
WHERE ancestry = 'Germany'
OR ancestry = 'France'
OR ancestry = 'England'
GROUP BY user_id
HAVING count(DISTINCT ancestry) = 3