Select a random sample of results from a query result

Question

This question asks about getting a random(ish) sample of records on SQL Server and the answer was to use TABLESAMPLE. Is there an equivalent in Oracle 10?

Answer 1

The SAMPLE clause will give you a random sample percentage of all rows in a table.

For example, here we obtain 25% of the rows:

SELECT * FROM emp SAMPLE(25)

The following SQL (using one of the analytical functions) will give you a random sample of a specific number of each occurrence of a particular value (similar to a GROUP BY) in a table.

Here we sample 10 of each:

SELECT * FROM (
SELECT job, sal, ROW_NUMBER()
OVER (
PARTITION BY job ORDER BY job
) SampleCount FROM emp
)
WHERE SampleCount <= 10