Question

I am not at all conversant in SQL so was hoping someone could help me with a query that will find all the records in a parent table for which there are no records in a child table.

The following works for me to find parent records for specific child field values…

`SELECT    ParentTable.ParentID
 FROM      ParentTable INNER JOIN
             ParentTable ON ParentTable.ParentID = ChildTable.ChildID
 WHERE     (ChildTable.ChildField_ = '2131')
 Group By
   ParentTable.ParentID
 Having
   count(distinct ChildTable.ChildField) > 0`

Can I change the where clause some how to find parent’s with a count of zero child records.

Thanks.

Answer 1

You can use a NOT EXISTS clause for this

SELECT ParentTable.ParentID
FROM ParentTable
WHERE NOT EXISTS (
    SELECT 1 FROM ChildTable
    WHERE ChildTable.ParentID = ParentTable.ParentID
)

There’s also the old left join and check for null approach

SELECT ParentTable.ParentID
FROM ParentTable
LEFT JOIN ChildTable
  ON ParentTable.ParentID = ChildTable.ParentID
WHERE ChildTable.ChildID IS NULL

Try both and see which one works better for you.