Denial of Service (DoS) attacks are a common method used to take down Websites, servers, or even sections of the Internet. DDoS creates a flood of traffic to a Website, server, or section of the internet that overwhelms it to a degree that it cannot function and eventually shuts down. The most infamous DDoS involving database servers was in 2003 when a computer worm called the SQL Slammer compromised more than 75,000 servers and slowed internet traffic. Unpatched databases were to blame, but typically, there is no way to completely prevent DDoS attacks.
Source: https://threatpost.com/lessons-wikileaks-ddos-attacks-122710/74805/