[R] Sampling from a database
Andrew Perrin
andrew_perrin at unc.edu
Fri Feb 1 14:54:33 CET 2002
The standard way to do this under PostgreSQL is:
SELECT * FROM tablename ORDER BY random() LIMIT n;
where n is the number of records you want returned.
----------------------------------------------------------------------
Andrew J Perrin - andrew_perrin at unc.edu - http://www.unc.edu/~aperrin
Assistant Professor of Sociology, U of North Carolina, Chapel Hill
269 Hamilton Hall, CB#3210, Chapel Hill, NC 27599-3210 USA
On Fri, 1 Feb 2002, F. Tusell wrote:
> I use RODBC and RpgSQL quite a lot to access files stored in another
> machine under PostgreSQL. Since I am now using files which do not fit
> into R's memory, I would like to take random samples. What I would
> like is to issue a query such as
>
> SELECT * FROM file WHERE runif > 0.9
>
> with "runif" being a uniformly distributed random number, generated on
> the fly; but I cannot find any way to do that in PostgreSQL. So far,
> my only idea is to create a new table with two fields, "IDnumber" and
> "Random", fill "IDnumber" with the similarly named field from the
> table to sample, populate "Random" with random numbers generated in R
> and then issue a query for records where "IDnumber" of the two tables
> match AND Random > 0.9.
>
> Seems to me clumsy, and surely there has to be a better way and
> someone knowing SQL can suggest it.
>
>
> ft.
> --
> Fernando TUSELL e-mail:
> Departamento de Econometría y Estadística etptupaf at bs.ehu.es
> Facultad de CC.EE. y Empresariales Tel: (+34)94.601.3733
> Avenida Lendakari Aguirre, 83 Fax: (+34)94.601.3754
> E-48015 BILBAO (Spain) Secr: (+34)94.601.3740
> PGP: finger etptupaf at bsdx01.bs.ehu.es http://etdx01.bs.ehu.es
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