Is there a succinct way to retrieve a random record from a sql server table?
I would like to randomize my unit test data, so am looking for a simple way to select a random id from a table. In English, the select would be "Select one id from the table where the id is a random number between the lowest id in the table and the highest id in the table."
I can't figure out a way to do it without have to run the query, test for a null value, then re-run if null.
Ideas?
Is there a succinct way to retrieve a random record from a sql server table?
Yes
SELECT TOP 1 * FROM table ORDER BY NEWID()
A NEWID() is generated for each row and the table is then sorted by it. The first record is returned (i.e. the record with the "lowest" GUID).
GUIDs are generated as pseudo-random numbers since version four:
The version 4 UUID is meant for generating UUIDs from truly-random or pseudo-random numbers.
The algorithm is as follows:
- Set the two most significant bits (bits 6 and 7) of the clock_seq_hi_and_reserved to zero and one, respectively.
- Set the four most significant bits (bits 12 through 15) of the time_hi_and_version field to the 4-bit version number from Section 4.1.3.
- Set all the other bits to randomly (or pseudo-randomly) chosen values.
—A Universally Unique IDentifier (UUID) URN Namespace - RFC 4122
The alternative SELECT TOP 1 * FROM table ORDER BY RAND() will not work as one would think. RAND() returns one single value per query, thus all rows will share the same value.
While GUID values are pseudo-random, you will need a better PRNG for the more demanding applications.
Typical performance is less than 10 seconds for around 1,000,000 rows — of course depending on the system. Note that it's impossible to hit an index, thus performance will be relatively limited.
Also try your method to get a random Id between MIN(Id) and MAX(Id) and then
SELECT TOP 1 * FROM table WHERE Id >= @yourrandomid
It will always get you one row.
If you really want a random sample of individual rows, modify your query to filter out rows randomly, instead of using TABLESAMPLE. For example, the following query uses the NEWID function to return approximately one percent of the rows of the Sales.SalesOrderDetail table:
SELECT * FROM Sales.SalesOrderDetail
WHERE 0.01 >= CAST(CHECKSUM(NEWID(), SalesOrderID) & 0x7fffffff AS float)
/ CAST (0x7fffffff AS int)
The SalesOrderID column is included in the CHECKSUM expression so that NEWID() evaluates once per row to achieve sampling on a per-row basis. The expression CAST(CHECKSUM(NEWID(), SalesOrderID) & 0x7fffffff AS float / CAST (0x7fffffff AS int) evaluates to a random float value between 0 and 1."
Source: http://technet.microsoft.com/en-us/library/ms189108(v=sql.105).aspx
This is further explained below:
How does this work? Let's split out the WHERE clause and explain it.
The CHECKSUM function is calculating a checksum over the items in the list. It is arguable over whether SalesOrderID is even required, since NEWID() is a function that returns a new random GUID, so multiplying a random figure by a constant should result in a random in any case. Indeed, excluding SalesOrderID seems to make no difference. If you are a keen statistician and can justify the inclusion of this, please use the comments section below and let me know why I'm wrong!
The CHECKSUM function returns a VARBINARY. Performing a bitwise AND operation with 0x7fffffff, which is the equivalent of (111111111...) in binary, yields a decimal value that is effectively a representation of a random string of 0s and 1s. Dividing by the co-efficient 0x7fffffff effectively normalizes this decimal figure to a figure between 0 and 1. Then to decide whether each row merits inclusion in the final result set, a threshold of 1/x is used (in this case, 0.01) where x is the percentage of the data to retrieve as a sample.