sql - UNION or Left Join? -

i have try union 2 tables , left join result no want.any idea ?please help.

table a

deduction_no  group_id    emp_id     service_code    amt  prd533       prg158      null          2351          35      prd533       prg158      null          4854          160          prd533       prg158      null          4857          44            prd533       prg158      null          6611           3               

table b

deduction_no  group_id    emp_id       is_selected    deduct_amt  prd533         prg158     001               y         82  prd533         prg158     001               n         0  prd533         prg158     002               y         195  prd533         prg158     002               n         0  prd533         prg158     003               y         79  prd533         prg158     003               n          0   prd533         prg158     004               y         107  prd533         prg158     004               n          0 

output,but don't want (union)

deduction_no  group_id    emp_id     service_code     deduct_atm  prd533       prg158      001          -               prd533       prg158      002          -               prd533       prg158      003          -              prd533       prg158      004          -                   prd533       prg158      -          2351               35  prd533       prg158      -          4854               160        prd533       prg158      -          4857               44  prd533       prg158      -          6611               3 

output using left join

deduction_no  group_id    emp_id     service_code     deduct_atm  prd533       prg158      001          2351             35  prd533       prg158      001          4854             160  prd533       prg158      001          4857             44  prd533       prg158      001          6611             3       prd533       prg158      002          2351             35  prd533       prg158      002          4854             160  prd533       prg158      002          4857             44  prd533       prg158      002          6611             3    

desired output

deduction_no  group_id    emp_id     service_code    amt  prd533       prg158      001          2351          35   prd533       prg158      002          2351          35    prd533       prg158      003          2351          35      prd533       prg158      001          4854          160          prd533       prg158      002          4857          44  prd533       prg158      004          4857          44             prd533       prg158      003          6611           3      prd533       prg158      004          6611           3   

thank you

this query:

 select a.deduction_no, a.group_id, b.emp_id, a.service_code, a.amt   #a   left join #b b on a.deduction_no = b.deduction_no   b.is_selected ='y'  order service_code 

will give following output:

deduction_no    group_id    emp_id  service_code    amt    prd533        prg158        1        2351         35    prd533        prg158        2        2351         35    prd533        prg158        3        2351         35    prd533        prg158        4        2351         35    prd533        prg158        1        4854         160    prd533        prg158        2        4854         160    prd533        prg158        3        4854         160    prd533        prg158        4        4854         160    prd533        prg158        1        4857         44    prd533        prg158        2        4857         44    prd533        prg158        3        4857         44    prd533        prg158        4        4857         44    prd533        prg158        1        6611         3    prd533        prg158        2        6611         3    prd533        prg158        3        6611         3    prd533        prg158        4        6611         3 

since don't give me more information can't you, should provide how data should selected, on conditions , etc.