Select n random rows from SQL Server table

Question

I've got a SQL Server table with about 50,000 rows in it. I want to select about 5,000 of those rows at random. I've thought of a complicated way, creating a temp table with a "random number" column, copying my table into that, looping through the temp table and updating each row with RAND(), and then selecting from that table where the random number column < 0.1. I'm looking for a simpler way to do it, in a single statement if possible.

This article suggest using the NEWID() function. That looks promising, but I can't see how I could reliably select a certain percentage of rows.

Anybody ever do this before? Any ideas?

Answer 1

It appears newid() can't be used in where clause, so this solution requires an inner query:

SELECT *
FROM (
    SELECT *, ABS(CHECKSUM(NEWID())) AS Rnd
    FROM MyTable
) vw
WHERE Rnd % 100 < 10        --10%

Answer 2

This works for me:

SELECT * FROM table_name
ORDER BY RANDOM()
LIMIT [number]

Answer 3

If you (unlike the OP) need a specific number of records (which makes the CHECKSUM approach difficult) and desire a more random sample than TABLESAMPLE provides by itself, and also want better speed than CHECKSUM, you may make do with a merger of the TABLESAMPLE and NEWID() methods, like this:

DECLARE @sampleCount int = 50
SET STATISTICS TIME ON

SELECT TOP (@sampleCount) * 
FROM [yourtable] TABLESAMPLE(10 PERCENT)
ORDER BY NEWID()

SET STATISTICS TIME OFF

In my case this is the most straightforward compromise between randomness (it's not really, I know) and speed. Vary the TABLESAMPLE percentage (or rows) as appropriate - the higher the percentage, the more random the sample, but expect a linear drop off in speed. (Note that TABLESAMPLE will not accept a variable)