1. Load the txt into a temporary table.
2. Use the "except" key word in sql server to get the records which are not existing in Actual table and existing in Temporary table.
e.g.
Select col1,col2, col3 from ActualTable
Except
Select col1,col2, col3 from TemporaryTable
3. Then insert the resultant records of step 2 into ActualTable.