For SQL Server it looks like an index seek on the following so it is evaluated once.

SELECT * FROM SomeTable WHERE ID = CAST(Rand() * 1000 as int)

In addition this query: SELECT TOP(100) rand() AS RandData from SomeTable

Returns: 0.940284064056996 0.940284064056996 0.940284064056996 . . . .

A old trick to actually get random data on SQL Server that I think has been replaced with newer functionality is:

SELECT ABS(CHECKSUM(NEWID())) FROM SomeTable

However if we move the NEWID.... stuff into the WHERE clause we end up switching to an index seek so back to being evaluated once.