Case with where clause - ORA-00920: Invalid relational operator

Question

Hi All, when I try to run the query below, I get the following error...
ORA-00920: invalid relational operator
00920. 00000 -  "invalid relational operator"
*Cause:   
*Action:
Error at Line: 16 Column: 5
Does anyone know what's wrong with my query? thanks in advance.
SELECT concat (year,period)
FROM DD_ACTUALS_FACT
WHERE CASE Period
WHEN 'JAN' THEN '01'
WHEN 'FEB' THEN '02'
WHEN 'MAR' THEN '03'
WHEN 'APR' THEN '04'
WHEN 'MAY' THEN '05'
WHEN 'JUN' THEN '06'
WHEN 'JUL' THEN '07'
WHEN 'AUG' THEN '08'
WHEN 'SEP' THEN '09'
WHEN 'OCT' THEN '10'
WHEN 'NOV' THEN '11'
WHEN 'DEC' THEN '12'
END as "MonthNo"
ORDER BY CONCAT (year,"MonthNo") DESC

Answer 1

The problem is the as "MonthNo" - you can't give an "AS" alias to an expression in a where clause.
You have not actually given any condition, just a set of translations from period into a number.
You also haven't said what you're trying to do.
Perhaps you want:
SELECT concat (year,period)
FROM DD_ACTUALS_FACT
WHERE something
ORDER BY CONCAT (year, CASE Period
WHEN 'JAN' THEN '01'
WHEN 'FEB' THEN '02'
WHEN 'MAR' THEN '03'
WHEN 'APR' THEN '04'
WHEN 'MAY' THEN '05'
WHEN 'JUN' THEN '06'
WHEN 'JUL' THEN '07'
WHEN 'AUG' THEN '08'
WHEN 'SEP' THEN '09'
WHEN 'OCT' THEN '10'
WHEN 'NOV' THEN '11'
WHEN 'DEC' THEN '12'
END  ) DESC