On Thursday, February 14, 2013 4:31 AM Andrew Taylor wrote:
> Hi,
> I'd like to do something which I think should be quite easy - that is =
join
2 tables and create a new table.
> Table A postcode_input has columns which include postcode, eastings,
northings. there are 1,687,605 rows.
> Table B bng_lat_long has columns lat, lon, e, n. There are 1,687,605 =
rows.
> eastings =3D e and northings =3D n so there should be a 1 to 1 match. =
The
eastings northings pair should be unique in=20
> aggregate.
I think mapping is m to n.
>So I tried doing this:
> SELECT A.postcode, A.eastings, A.northings, B.lat, B.lon INTO
postcode_lat_long
> FROM postcode_input AS A
> LEFT JOIN bng_lat_long AS B On A.eastings =3D B.e AND A.northings =3D =
B.n
> And ended up with a table 13,708,233 rows long with what looks like =
plenty
of duplicated rows. Some but not all are=20
> duplicated. What can I do to sort this out?=A0
What is you exact expection of data in postcode_lat_long?
From the above it seems you want distinct rows which match between
postcode_input and bng_lat_long, if there is no match then take the =
values
of postcode_input and NULL for bng_lat_long.
If I am right, then you can try with using DISTINCT operator:
http://www.postgresql.org/docs/9.2/static/sql-select.html#SQL-DISTINCT
With Regards,
Amit Kapila.
