You can use TABLESAMPLE, like this:
FROM YourTable TABLESAMPLE (2.5 PERCENT)
However, BOL notes the following limitations of TABLESAMPLE:
You can use TABLESAMPLE to quickly return a sample from a large table when either of the following conditions is true:
The sample does not have to be a truly random sample at the level of individual rows.
Rows on individual pages of the table are not correlated with other rows on the same page.
If you really need a "random" sample then BOL suggests this:
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.
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