query to subtract date from systimestamp in oracle 11g - sql

Question

I want to perform a subtraction operation on the date returned from another query and the system time in oracle SQL. So far I have been able to use the result of another query but when I try to subtract from systimestamp it gives me the following error
ORA-01722: invalid number
'01722. 00000 - "invalid number"
*Cause: The specified number was invalid.
*Action: Specify a valid number.
Below is my query
select round(to_number(systimestamp - e.last_time) * 24) as lag
from (
select ATTR_VALUE as last_time
from CONFIG
where ATTR_NAME='last_time'
and PROCESS_TYPE='new'
) e;
I have also tried this
select to_char(sys_extract_utc(systimestamp)-e.last_time,'YYYY-MM-DD HH24:MI:SS') as lag
from (
select ATTR_VALUE as last_time
from CONFIG
where ATTR_NAME='last_time'
and PROCESS_TYPE='new'
) e;
I want the difference between the time intervals to be in hours.
Thank you for any help in advance.
P.S. The datatype of ATTR_VALUE is VARCHAR2(150). A sample result of e.last_time is 2016-09-05 22:43:81796

Answer 1

"its VARCHAR2(150). That means I need to convert that to date"
ATTR_VALUE is a string so yes you need to convert it to the correct type before attempting to compare it with another datatype. Given your sample data the correct type would be timestamp, in which case your subquery should be:
(
select to_timestamp(ATTR_VALUE, 'yyyy-mm-dd hh24:mi:ss.ff5') as last_time
from CONFIG
where ATTR_NAME='last_time'
and PROCESS_TYPE='new'
)
The assumption is that your sample is representative of all the values in your CONFIG table for the given keys. If you have values in different formats your query will break on some other way: that's the danger of using this approach.

Answer 2

So finally after lots of trial and errors I got this one
1. Turns out initially the error was because the data_type of e.last_time was VARCHAR(150).
To find out the datatype of a given column in the table I used
desc <table_name>
which in my case was desc CONFIG
2. To convert VARCHAR to system time I have two options to_timestamp and to_date. If I use to_timestamp like
select round((systimestamp - to_timestamp(e.last_time,'YYYY-MM-DD HH24:MI:SSSSS')) * 24, 2) as lag
from (
select ATTR_VALUE as last_time
from CONFIG
where ATTR_NAME='last_time'
and PROCESS_TYPE='new'
) e;
I get an error that round expects NUMBER and got INTERVAL DAY TO SECONDS since the difference in date comes out to be like +41 13:55:20.663990. To convert that into hour would require a complex logic.
An alternative is to use to_data which I preferred and used it as
select round((sysdate - to_date(e.last_time,'YYYY-MM-DD HH24:MI:SSSSS')) * 24, 2) as lag
from (
select ATTR_VALUE as last_time
from CONFIG
where ATTR_NAME='last_time'
and PROCESS_TYPE='new'
) e;
This returns me the desired result i.e. the difference in hours rounded off to 2 floating digits