Can I use an expression in order by clause

Question

SELECT
PERIOD_NUM, period_num *12,
period_name,
--TO_DATE(SUBSTR(PERIOD_NAME,1,3)| |SUBSTR(PERIOD_NAME,INSTR(PERIOD_NAME,'-'),3),'MON-RR'),
TO_NUMBER(TO_CHAR(TO_DATE((SUBSTR(PERIOD_NAME,1,3)| |SUBSTR(PERIOD_NAME,INSTR(PERIOD_NAME,'-')+1,2)),'MONRR'),'RRRRMM'))
FROM GL.GL_PERIODS
order by 2--TO_NUMBER(TO_CHAR(TO_DATE(SUBSTR(PERIOD_NAME,1,3)| |SUBSTR(PERIOD_NAME,INSTR(PERIOD_NAME,'-')+1,2),'MONRR'),'RRRRMM'))
desc
gl_periods is a table, period_name consists of values like oct-01_fy-01 etc and oct01_01_fy-01
I want to sort on this column based on period_name, so I am trying to convert into number like for oct-01_fy-01, the query gives
200110 and so on. the recent period comes first, But my question is I can't use the same to_number(.....) in the order by clause. oracle gives an error
ORA-01843: not a valid month.
Let me know whether anyone come across with this kind of situation

Answer 1

If you are sure the expression is correct, since you are using it in the select clause, give an alias and use the alias in the order by clause.
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