MySQL Query to find customers who have ordered two specific products

Question

I\'m having trouble coming up with a query that will find all customers who have purchased both PROD1 and PROD2.

Here\'s a pseudo-query that kind of looks like what

Answer 1

SELECT COUNT(DISTINCT userId)
FROM(
  SELECT userId
  FROM transactions
  WHERE product = 'PROD1'
  INTERSECT
  SELECT userId
  FROM transactions
  WHERE product = 'PROD2');

The query creates two intermediate tables, one which contains userId of customer who bought PROD1 and another identical table for those who bought PROD2. The intersection operator returns a table which contains only rows found in both previous tables, i.e those who bought both products.