Why Can't I Directly Compare Aliases in a WHERE Clause, and How Can I Work Around This?

WHERE Clause Alias Limitations and Solutions

Problem:

When querying table_a for records not recently updated, using aliases in the WHERE clause results in an "ORA-00904 Invalid Identifier" error. This occurs because the alias MONTH_NO is not yet defined when the WHERE clause is processed.

Here's an example:

SELECT A.identifier
     , A.name
     , TO_NUMBER(DECODE( A.month_no, 1, 200803, ..., 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

Solution:

The WHERE clause executes before the SELECT clause, preventing direct alias usage. The solution is to use a subquery:

SELECT * FROM
(
  SELECT A.identifier
    , A.name
    , TO_NUMBER(DECODE( A.month_no, 1, 200803, ..., 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

This approach first defines the aliases within the inner query. The outer query then references these already-defined aliases, resolving the "invalid identifier" error. The SELECT * in the outer query selects all columns from the inner query's result set.

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