{
sqlplus un/pw <<EOF
select col_a
       , col_b
from   table_a a
where exists (
      select 1 
      from   table_b b 
      where a.col_a = b.col_b
);
EOF
} | while read COL_A COL_B
do
   echo $COL_A $COL_B
done

{
sqlplus un/pw <<EOF
select col_a
from   table_a a;
EOF | while read COL_A
do
    sqlplus un/pw <<EOF
select col_c
from   table_b
where col_a = ${COL_A};
EOF 
} | while read COL_B
    do
        echo ${COL_B}
    done
done

{
typeset IFS='
'
sqlplus -s un/pw <<EOF
select t.table_name, c.column_name
 from
   (select table_name
          ,column_name
    from   all_tab_columns) c
  ,(select table_name
    from   all_tables) t
where t.table_name = c.table_name
and   t.table_name = 'PLAN_TABLE'
;
EOF
} | while read line
do
    echo "$line"
done

TABLE_NAME                     COLUMN_NAME
------------------------------ ------------------------------
PLAN_TABLE                     STATEMENT_ID
PLAN_TABLE                     TIMESTAMP
PLAN_TABLE                     REMARKS
PLAN_TABLE                     OPERATION
PLAN_TABLE                     OPTIONS
PLAN_TABLE                     OBJECT_NODE
PLAN_TABLE                     OBJECT_OWNER
...

27 rows selected.