Looping through database columns and updating new

Question

Hi new to SQL.
I have table_a with 2 columns called Column_1 and Column_2 - plus another table_b with Column_3. I want to read every entry in table_b Column_3
because it could be in table_a column_2 multiple times. If it is then I want to insert table_a column_1 value plus table_b Column_3 value into a new table
called table_c with these 2 new columns.
thanks

Answer 1

A bulk insert (if you can work out the logic) should be more efficent (execute faster) than a cursor loop with an insert - BUT it might be easier to do the loop with the insert with less time to develop if performance isn't too bad. The bulk insert will avoid context switching between PL/SQL and SQL which can use a lot of resources.
The bulk insert could look something like (you will have to iron out the lookup logic yourself)
insert into new_table (col1, col2)
  select col1, col2 + col3
    from table_a a1, table b
   where a1.key = b.key
      and exists(
        select 0
          from table_a a2
         where a2.col1 = a1.col1
       group by col1
       having count(col1) > 1
       )