Question

I have a one to many relation that looks like this:

| Parent |  | Child  |
|   id   |  |   id   |
|        |  |parentID|
|        |  |  date  |

And I am trying to structure a query such that I get all of the parents who have children records which ALL have a date before a specified date.

Something like this

SELECT * FROM parent
JOIN child on child.parentid = parent.id
WHERE child.date <= '10/13/2010'

But the problem with this is I get parents that have children with a date before the date specified and have child records with a date after the date specified, when I want ONLY the parents of children with a date before the given date.

Does anyone have some suggestions on how to handle this case?

Thanks!

Answer 1

Use:

SELECT p.*
  FROM PARENT p
 WHERE EXISTS(SELECT NULL
                FROM CHILD c
               WHERE c.parentid = p.id
                 AND c.date <= '2010-10-13')
   AND NOT EXISTS(SELECT NULL
                    FROM CHILD c
                   WHERE c.parentid = p.id
                     AND c.date > '2010-10-13')

Everyone will tell you to use JOINs “because they’re faster”, but typically they aren’t aware of the impact of using them — if you don’t need the information from a supporting table, you shouldn’t be joining to it. That’s because more than one child in this situation would produce duplicate PARENT records. The trade-off between a JOIN and DISTINCT or GROUP BY vs IN or EXISTS is probably par, but without the hassle of dealing with the duplicated data properly.