oracle - Is it possible to use both DECODE and SELECT STATEMENT? -

i'm new in using sql loader. possible use both decode , select statement? i'm trying work kind of logic , fail solve it.

dummy_field  "decode(a, null, (select item_here db_table column=:dummy_field), a)", 

i missing expression error. tried inserting expression select statement , no luck.

if want join incoming data other tables i'd suggest using external table instead of sql*loader, external table's data can joined other data, aggregated, ordered , otherwise manipulated if selected normal table.

also:

decode(a, null, expr, a) 

... better expressed ...

coalesce(a, expr)