Question

I have a table “Table” similar to this one:

id     user_id      owner_id      due_date
------+------------+-------------+--------------------------
1     | 1          | 1           | 2011-07-26 12:28:50
2     | 1          | 1           | 2011-07-26 15:32:11
3     | 1          | 1           | 2011-07-27 08:11:58
4     | 2          | 1           | 2011-07-26 15:19:44
5     | 2          | 1           | 2011-07-23 12:00:50

As you can see, a user identified by FK user_id can have multiple entries with different due_date.

I would like to fetch all most recent Table entities grouped by user_id.

In plain MySQL, it could look like this:

SELECT * FROM (
    SELECT * FROM TABLE
      WHERE owner_id = xxx
      ORDER BY due_date DESC
  ) AS sorted
GROUP BY user_id

I would first select with ORDER BY, then apply a GROUP BY (all in one SELECT doesn’t work, unfortunately. see e.g. here http://www.cafewebmaster.com/mysql-order-sort-group).

Now I would like to implement this as HQL (with Grails). The problem is that HQL does not support inner SELECT’s in the FROM part, so the following approach won’t work:

def findMostRecentPerUser( Owner owner ) {
    def result = Table.executeQuery("""
                from Table as t1
                where t1.id in (
                    select sorted.id from (select * from Table as t2 where t2.owner_id = ${owner.id} order by t2.due_date desc) as sorted group by sorted.user_id
            )
        """)
    return result
}

Another way would be a Criteria but I’ve no idea how to implement it that way.

Could someone point me into the right direction? Your help is greatly appreciated.

Answer 1

This way should work. Improvements appreciated.

def findMostRecentPerUser( Owner owner ) {
    // Inner query sorts maximum due_date per user
    // Outer query fetches all Table entries with this due_date
    def result = Table.executeQuery("""
            from Table as t1
            where t1.due_date in (
                select max(t2.due_date) from Table as t2 where t2.owner = :owner group by t2.user
            )
        """,
        [ owner: owner ] )
    return result
}