If you do a search for:
http://www.google.co.uk/search?q=0x57414954464F522044454C4159202730303A30303A313527&hl=en&start=30&sa=N
you will see a lot of examples of an attempted hack along the lines of:
1) declare @q varchar(8000) select @q = 0x57414954464F522044454C4159202730303A30303A313527 exec(@q) --
What is exactly is it trying to do? Which db is it trying to work on?
Do you know of any advisories about this?
He is testing your server for SQL Injection, specifically this is a robust test that will work even if its Blind SQL Injection. Blind SQL Injection is when an attacker is able to execute SQL however, there isn't a viewable response. If the http request takes at least 15 seconds the attacker will know that he can execute SQL, and that your running MS-SQL. After this attack he will follow it up with a xp_cmpdshell() to infect your server.
According to http://bytes.com/topic/mysql/answers/888849-hacker-attempt it looks like it's trying to run:
WAITFOR DELAY '00:00:15'
As others have pointed out it's not a DOS attack (as I originally stated) but merely a way to easily determine if the SQL Server is vulnerable and can be added to a list of hosts to perhaps further hack away at later on.
In simpler terms he/she/it is very slick. Using the "WAITFOR DELAY..." strategy allows he/she/it to see if the server is vulnerable without logging anything. The check is being done to see what access the connectionstring user has in the db. And like @Rook said, thT WOULD LEAD TO XP_CMDSHELL() which can give the intruder access to the server and even your network.
That is a hex string. When you translate it, it translates into: "WAITFOR DELAY '00:00:15'"