Обсуждение: Question on Select

Hi,
I have two tables,
  Table A: item_id
  Table B: item_id,  ref_code
and i want to list  all ref_codes in table B that are not referenced by
Table A.
Table A has about 3million records./ table B 200

What is the best way to do that ?

Thanks
Alex

select ref_code from tab_b where not exists (select * from tab_a where
item_id = tab_b.item_id);

make sure u have index on item_id  in tab_a .

the above sql is quite efficient , other method is to use left join.


Alex wrote:

> Hi,
> I have two tables,
>  Table A: item_id
>  Table B: item_id,  ref_code
> and i want to list  all ref_codes in table B that are not referenced
> by Table A.
> Table A has about 3million records./ table B 200
>
> What is the best way to do that ?
>
> Thanks
> Alex
>
>
>
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