How to Resolve 'ORA-00904 Invalid Identifier' Error When Using Aliases in a WHERE Clause?

Resolve the error of invalid alias in WHERE clause

When using an alias in a WHERE clause, such as the following example, an "ORA-00904 invalid identifier" error may occur:

SELECT *
FROM table_a A
JOIN table_b B ON A.identifier = B.identifier
WHERE MONTH_NO > UPD_DATE

This error occurs because when using aliases (such as MONTH_NO and UPD_DATE) directly in the WHERE clause, they are not referenced in the SELECT list.

Error reason:

During query execution, the WHERE clause is evaluated before the SELECT list. Therefore, Oracle cannot recognize the alias in the WHERE clause until the alias is defined in the SELECT list.

Solution:

There are two main ways to solve this problem:

1. Subquery and filtering:

Create a subquery that contains the necessary calculations and aliases, then filter based on the aliases in the outer query.

SELECT *
FROM
(
  SELECT A.identifier, A.name,
    TO_NUMBER(DECODE(A.month_no, ...)) AS MONTH_NO,
    TO_NUMBER(TO_CHAR(B.last_update_date, 'YYYYMM')) AS UPD_DATE
  FROM table_a A
  JOIN table_b B ON A.identifier = B.identifier
) AS inner_table
WHERE MONTH_NO > UPD_DATE

2. Common table expression (CTE):

Define a CTE to create a temporary table containing the necessary calculations and aliases, and then reference the CTE in the outer query.

WITH tmp_data AS (
  SELECT A.identifier, A.name,
    TO_NUMBER(DECODE(A.month_no, ...)) AS MONTH_NO,
    TO_NUMBER(TO_CHAR(B.last_update_date, 'YYYYMM')) AS UPD_DATE
  FROM table_a A
  JOIN table_b B ON A.identifier = B.identifier
)
SELECT *
FROM tmp_data
WHERE MONTH_NO > UPD_DATE

Performance Notes:

There are many optimizations in the current Oracle version. Oracle does not require internal queries to be materialized before applying external conditions. The server will push down the predicate for cost-effective execution.

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