Question

I have a query which is meant to show me any rows in table A which have not been updated recently enough. (Each row should be updated within 2 months after ‘month_no’.):

SELECT A.identifier      , A.name      , TO_NUMBER(DECODE( A.month_no              , 1, 200803               , 2, 200804               , 3, 200805               , 4, 200806               , 5, 200807               , 6, 200808               , 7, 200809               , 8, 200810               , 9, 200811               , 10, 200812               , 11, 200701               , 12, 200702              , NULL)) as MONTH_NO      , TO_NUMBER(TO_CHAR(B.last_update_date, 'YYYYMM')) as UPD_DATE   FROM table_a A      , table_b B  WHERE A.identifier = B.identifier    AND MONTH_NO > UPD_DATE 

The last line in the WHERE clause causes an ‘ORA-00904 Invalid Identifier’ error. Needless to say, I don’t want to repeat the entire DECODE function in my WHERE clause. Any thoughts? (Both fixes and workarounds accepted…)

Answer 1

This is not possible directly, because chronologically, WHERE happens before SELECT, which always is the last step in the execution chain.

You can do a sub-select and filter on it:

SELECT * FROM (   SELECT A.identifier     , A.name     , TO_NUMBER(DECODE( A.month_no       , 1, 200803        , 2, 200804        , 3, 200805        , 4, 200806        , 5, 200807        , 6, 200808        , 7, 200809        , 8, 200810        , 9, 200811        , 10, 200812        , 11, 200701        , 12, 200702       , NULL)) as MONTH_NO     , TO_NUMBER(TO_CHAR(B.last_update_date, 'YYYYMM')) as UPD_DATE   FROM table_a A     , table_b B   WHERE A.identifier = B.identifier ) AS inner_table WHERE    MONTH_NO > UPD_DATE 

Interesting bit of info moved up from the comments:

There should be no performance hit. Oracle does not need to materialize inner queries before applying outer conditions — Oracle will consider transforming this query internally and push the predicate down into the inner query and will do so if it is cost effective. – Justin Cave