Convert result of CASE Expression to a number in Oracle SQL

I have the following select query. I want to convert it from string into a number. If I wrap a TO_NUMBER around the case expression, I get

expression must have same datatype as corresponding expression

error.

SELECT CASE SUBSTR(GRADE, INSTR(GRADE, ' ') + 1) 
         WHEN 'Unspecified' THEN ' '
         ELSE SUBSTR(GRADE, INSTR(GRADE, ' ') + 1) 
       END as Final_Grade,

How can I get Final_Grade to be numeric?

Thank you!

Answer

Well, ' ' is not a number, so better figure out what to do. I would suggest NULL:

SELECT (CASE SUBSTR(GRADE, INSTR(GRADE, ' ') + 1) 
           WHEN 'Unspecified' THEN NULL
           ELSE TO_NUMBER(SUBSTR(GRADE, INSTR(GRADE, ' ') + 1)) 
        END) as Final_Grade,

Actually, I prefer:

(CASE WHEN GRADE NOT LIKE 'Unspecified%'
      THEN TO_NUMBER(SUBSTR(GRADE, INSTR(GRADE, ' ') + 1)) 
 END) as Final_Grade

Or perhaps even more safely:

(CASE WHEN REGEXP_LIKE(GRADE, '^[0-9]+ ')
      THEN TO_NUMBER(SUBSTR(GRADE, INSTR(GRADE, ' ') + 1)) 
 END) as Final_Grade