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Finding rows in parent tables with no corresponding entries in child table

I have two tables with a one-to-many relationship. How can I find the rows in my parent table that don't have a corresponding entry in my child table? Thanks!

This one is pretty straightforward; the EXISTS operator and a correlated sub-query work well here:

select * from ParentTable pt
  where not exists ( select * from ChildTable ct
                       where ct.ParentID = pt.ParentID )

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