hi, I have two tables with following definition

    TableA                                          TableB
ID1   ID2   ID3  Value1 Value                      ID1 Value1
 C1    P1   S1                                     S1
 C1    P1   S2                                     S2
 C1    P1   S3                                     S3
 C1    P1   S5                                     S4
                                                   S5

The values are just examples in the table. TableA has a clustered primary key ID1, ID2 & ID3 and TableB has p.k. ID1 I need to create a table that has the missing records in TableA based on TableB The select query I am trying to create should give the following output

C1  P1  S4

To do this, I have the following SQL query

    SELECT
    DISTINCT TableA.ID1, TableA.ID2, TableB.ID1
    FROM TableA a, TableB b 
    WHERE TableB.ID1 NOT IN 
    ( 
      SELECT DISTINCT [ID3] 
      FROM TableA aa
      WHERE a.ID1 == aa.ID1 
        AND a.ID2 == aa.ID2 
    )

Though this query works, it performs poorly and my final TableA may have upto 1M records. is there a way to rewrite this more efficiently.

Thanks for any help, Javid