Need SQL Query to find Parent records without child records

Question

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.