can I use the to_char function in where clause in a query with PL/SQL?

Question

I am trying to find out average of electricity volume of certain day in a week with the following query:

SELECT avg(volume)
FROM v_nem_rm16
WHERE to_char(day, 'day') = 'monday';

where the v_nem_rm16 is a table and volume, day are its columns and my query is returning null whatever I change the day value 'monday', 'tuesday',....

is this query wrong?

Answer 1

Actually 'DAY' is returned with padding spaces on the right side. If you use 'RTRIM' then you can avoid the null values.

SELECT avg(volume)
FROM v_nem_rm16
WHERE RTRIM(to_char(day, 'day')) = 'monday';

Answer 2

I would rather use different date format to_char DAY is nls-dependent that is bad (for instance your software will fail in Spain). D returns number so in your case the query should look like

SELECT avg(volume)
FROM v_nem_rm16
WHERE RTRIM(to_char(day, 'd')) = 1;