I got this right, but the explanation, to me, doesn't really sound right:
"TABLESAMPLE returns an approximate percentage of rows, even if a number of rows is specified. This is used to get a sample of data from large rows and does not guarentee a number of rows or a random sample"
From http://technet.microsoft.com/en-us/library/ms189108.aspx, it says TABLESAMPLE returns a sample number of rows from the result set, so if 10 percent is used, then 10 percent is returned. Quote from the link says:
The TABLESAMPLE clause limits the number of rows returned from a table in the FROM clause to a sample number or PERCENT of rows.
However, if SYSTEM is used, then if 10 percent is specified, then around 10 percent of the result set is returned. Quote from the link says:
TABLESAMPLE SYSTEM returns an approximate percentage of rows and generates a random value for each physical 8-KB page in the table.....For example, if you specify TABLESAMPLE SYSTEM 10 PERCENT, SQL Server returns all the rows on approximately 10 percent of the specified data pages of the table.....However, because the random value that is generated for each page is independent of the values that are generated for any other page, a larger, or smaller, percentage of pages than have been requested might be returned.
Am I reading that right?