Question

I’d like to convert this problem into a query.
At a guess should be three leftjoin and two select

exercises
+-----+---------+
|  id | require | 
+-----+---------+
|  1  |         |
|  2  |    1    |
|  3  |    2    |
|  4  |    3    |
|  5  |         |
|  6  |    5    |
|  7  |         |
+-----+---------+

done
+-----+
|  id |
+-----+
|  1  |
|  2  |
+-----+

Having done the exercises one and two, which exercises can I do now?
Solution: 3,5,7

My logical process, step by step:
(all) – SELECT(required – done)-(done)
(1,2,3,4,5,6,7) – SELECT((1,2,3,5)-(1,2)) – (1,2)
(1,2,3,4,5,6,7) – SELECT(3,5) – (1,2)
(1,2,3,4,5,6,7) -(4,6) – (1,2)
(3,5,7)

How to write the query?

Answer 1

Another solution without subselect

SELECT e.id FROM exercises e 
LEFT JOIN done d ON d.id = e.id 
LEFT JOIN done d_req ON d_req.id=e.require 
WHERE (d.id IS NULL AND d_req.id IS NOT NULL) 
         OR (e.require IS NULL AND d.id IS NULL)

the WHERE can be semplified (distributivity)

WHERE d.id IS NULL AND (d_req.id IS NOT NULL OR e.require IS NULL )