SQL Group by仅在满足条件的情况下

Question

I am trying to query few users from a table joined to a bookings table but there are conditions to satisfy and I don't know how to get the expected results.

Conditions

• Return unique users
• Only if lasts 3 completed bookings are eq to notified is null

Tables

Users

id | name
---+-----
1  | Jonh
2  | Jim
3  | Jen

Bookings

id | user_id |    state    |        notified
---+---------+-------------+----------------------
12 | 1       | 'completed' | NULL
11 | 2       | 'completed' | NULL
10 | 3       | 'completed' | '2018-01-01 00:00:00'
9  | 1       | 'completed' | '2018-01-01 00:00:00'
8  | 2       | 'completed' | NULL
7  | 3       | 'completed' | NULL
6  | 1       | 'completed' | '2018-01-01 00:00:00'
5  | 2       | 'completed' | NULL
4  | 3       | 'completed' | NULL
3  | 1       | 'completed' | '2018-01-01 00:00:00'
2  | 2       | 'completed' | '2018-01-01 00:00:00'
1  | 3       | 'completed' | NULL

Query a have so far

SELECT users.id, bookings.id, bookings.notified
FROM users
  JOIN bookings ON users.id = bookings.user_id
  WHERE bookings.state = 'completed'
GROUP BY users.id, bookings.id, bookings.notified
HAVING (bookings.notified IS NULL)
ORDER BY bookings.id DESC;

Fiddle LINK

Expected

user_id 
-------
2

Answer 1

The following query returns the users that you want:

SELECT b.user_id
FROM bookings b
WHERE b.id >= (SELECT b2.id
               FROM bookings b2
               WHERE b2.user_id = b.user_id AND b2.state = 'completed'
               ORDER BY b2.id DESC
               OFFSET 2 FETCH FIRST 1 ROW ONLY
              ) AND
     b.state = 'completed'
GROUP BY b.user_id
HAVING COUNT(notified) = 0;

Here is the SQL Fiddle. Note this SQL Fiddle uses Postgres not MySQL (yours uses MySQL).