Question

Is there a sql query that can generate the date when 2 things appear together?
I mean, let’s say I have a table consists of bus schedule. Then, I have bus A and B. Bus A will operate on 22 May, 24 May, and 25 May while B operates on 22 May, 24 May and 26 May. I want to get the most recent date that 2 buses appear together which is 24 May.

Answer 1

To see those that both buses share:

  SELECT t.date
    FROM YOUR_TABLE t
   WHERE t.bus IN ('A', 'B')
GROUP BY t.date
  HAVING COUNT(DISTINCT t.bus) = 2

To see the most recent date that both buses share:

  SELECT t.date
    FROM YOUR_TABLE t
   WHERE t.bus IN ('A', 'B')
GROUP BY t.date
  HAVING COUNT(DISTINCT t.bus) = 2
ORDER BY t.date DESC
   LIMIT 1