Correlated Subquery vs Nested query

Hi,

I have a couple of questions below.

1) Which one is faster?
correlated subquery or nested query.

2) Which one is efficient?
SQL*Loader or UTL_FILE

3) Can I use DECODE function in a query within PL/SQL block.

I appreciate your reply for the above.

Thanks.

RaviShankar.

try by urself, will know better.