oracle sql basics

Display the values ​​of the two fields in one column
select u.id || u.name from user

Replace with u characters, if this field is empty, display 0
nvl(field name, 0)

Check the sql
select sql_text from v in user execution $session a, v$sqltext b where a.sql_address = b.address and username = 'newhyfx' order by address, piece;