This is obviously an DoS attack on the MySQL database, someone is obviously disrupting the server by sending more requests than it can possibly handle, preventing access to the site, this will pass, If its a SYN flood the send sends a flood of TCP/SYN packets, often with a forged sender address. Each of these packets are handled like a connection request, causing the server to spawn a half-open connection, by sending back a TCP/SYN-ACK packet, and waiting for a TCP/ACK packet in response from the sender address, SYN cookies modify the TCP protocol handling of the server by delaying allocation of resources until the client address has been verified. This seems to be the most powerful defense against SYN attacks. But the most likely reason of all is some script kiddies using some tools they have found on google, if you can trace the IP's and report to there ISP they could be convicted of breaking an internet law and have the internet service cut off completely.
Im guessing its also a group of disgruntled members who have been banned recently and are trying to get there revenge back, go through your recently banned and from when your first attacks occurred, try trace the IP of the attacks and if any match report them to there ISP and let the ISP do the rest.