I have three tables,
1. Imports (field of XNumber)
2. General Info (fields of XNumber, YNumber)
3. Milestones (field of YNumber)

and what I need to do is delete records from the Milestones table where the Imports key/field doesn't match. So, basically delete all unmatched records in Milestones where there's no match with Imports.

The problem is that I've got to use General Info as a join, or parent table.
The schema looks like (at least I'm trying this way):
GeneralInfo.YNumber TO Milestones.YNumber
GeneralInfo.XNumber TO Imports.XNumber

YNumber in the GeneralInfo table is the unique identifer, and I do have this to work with on a select or delete statement.

There can be multiple same YNumbers in Milestones, and multiple same XNumbers in Imports.