Using ‘case expression column’ in where clause

The reason for this error is that SQL SELECT statements are logically * processed in the following order:

• FROM: selection of one table or many JOINed ones and all rows combinations that match the ON conditions.

• WHERE: conditions are evaluated and rows that do not match are removed.

• GROUP BY: rows are grouped (and every group collapses to one row)

• HAVING: conditions are evaluated and rows that do not match are removed.

• SELECT: list of columns is evaluated.

• DISTINCT: duplicate rows are removed (if it’s a SELECT DISTINCT statement)

• UNION, EXCEPT, INTERSECT: the action of that operand is taken upon the rows of sub-SELECT statements. For example, if it’s a UNION, all rows are gathered (and duplicates eliminated unless it’s a UNION ALL) after all sub-SELECT statements are evaluated. Accordingly for the EXCEPT or INTERSECT cases.

• ORDER BY: rows are ordered.

Therefore, you can’t use in WHERE clause, something that hasn’t been populated or calculated yet. See also this question: oracle-sql-clause-evaluation-order

^{* logically processed:} Note that database engines may as well choose another order of evaluation for a query (and that’s what they usually do!) The only restriction is that the results should be the same as if the above order was used.

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Solution is to enclose the query in another one:

SELECT *
FROM
  ( SELECT ename
         , job
         , CASE deptno
             WHEN 10 THEN 'ACCOUNTS'
             WHEN 20 THEN 'SALES'
                     ELSE 'UNKNOWN'
           END AS department
    FROM emp
  ) tmp
WHERE department="SALES" ;

or to duplicate the calculation in the WHERE condition:

SELECT ename
     , job
     , CASE deptno
         WHEN 10 THEN 'ACCOUNTS'
         WHEN 20 THEN 'SALES'
                 ELSE 'UNKNOWN'
       END AS department
FROM emp
WHERE
    CASE deptno
      WHEN 10 THEN 'ACCOUNTS'
      WHEN 20 THEN 'SALES'
              ELSE 'UNKNOWN'
    END = 'SALES' ;

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I guess this is a simplified version of your query or you could use:

SELECT ename
     , job
     , 'SALES' AS department
FROM emp
WHERE deptno = 20 ;