Don't be afraid of breaking Oracle by actually trying things.
Break it into pieces and look at each piece one at a time.
Remove the TRUNC operation and see what you get. Do you understand why you get that result? If not, post it and we can help explain it.
But you need to make the effort yourself.
See the SQL Language doc. It has an example just like yours.
n rounded to
integer places to the right of the decimal point. If you omit
n is rounded to zero places. If
integer is negative, then
n is rounded off to the left of the decimal point.
The following example rounds a number one digit to the left of the decimal point:
SELECT ROUND(15.193,-1) "Round" FROM DUAL; Round ---------- 20
Which means your number, 156,00, will get rounded one digit to the left of the decimal point. So 156 gets rounded to 160.00 since the '6' is equal to or larger than '5' it gets rounded up.
That leaves a result of 160.00. The TRUNC operation does something similar excepts instead of rounding one place left it truncates. But since one place left already has a '0' there is nothing for TRUNC to do so you still have 160.00
Let me correct it. I know the output is 160, but wants to understand how it's done. I read the oracle documentation and understand how ROUND and TRUNC works, but I can't understand the result.
If you've read the manual, then you can ask a question like "Why does this query return 160? I expected it to return ... because, in the section on ROUND, the manual says ... so I expect ROUND to return ... but when I run ROUND by itself, I get ...". That way, you can get an answer that really focuses on what you need to know.