CodeListing 4: LAG analysis showing missing months

        PRODUCT_NAME  DATE_OF_SALE  LAST_MONTH  TOT_SALES  LAST_MONTHS_SALES
        ------------  -----------   ---------   --------  ----------------
        TV              01-JUL-03                   496
        TV              01-AUG-03   01-JUL-03       993      496     
        TV              01-OCT-03   01-AUG-03       468      993
        TV              01-NOV-03   01-OCT-03       535      468
        TV              01-DEC-03   01-NOV-03       665      535
        VCR             01-JUL-03                   617
        VCR             01-AUG-03   01-JUL-03       984      617
        VCR             01-SEP-03   01-AUG-03       308      984
        VCR             01-OCT-03   01-SEP-03       954      308
        VCR             01-DEC-03   01-OCT-03       947      954

        10 rows selected