Random Sampling in Google BigQuery

Raymond Raymond visibility 2,078 event 2021-05-17 access_time 3 years ago language English

In databases like Teradata, SAMPLE clause can be used to sample data from a large data table. Google BigQuery SQL didn't support SAMPLE clause at the very beginning however now TABLESAMPLE clause is added to support querying random sample subset of a large data set.

Use RAND function

Before TABLESAMPLE is added, RAND function is used as an alternative to retrieve random sample subset. The querying cost is big as the whole table will be scanned to generate one random number for each record. 


SELECT col1, col2, RAND(5) AS rnd FROM `myproj.mydataset.mytable` ORDER BY rnd LIMIT 10;

The above query returns 10 random records.


TABLESAMPLE clause is used to sampling a subset from a large data set. 


SELECT * FROM `myproj.mydataset.mytable` TABLESAMPLE SYSTEM (5 PERCENT)

The above example samples 5 percent of the data available in the specified table.  The clause doesn't support specifying records volume. For more information about table sampling, refer to Table sampling  |  BigQuery  |  Google Cloud.

More from Kontext
info Last modified by Raymond 3 years ago copyright This page is subject to Site terms.
comment Comments
No comments yet.

Please log in or register to comment.

account_circle Log in person_add Register

Log in with external accounts