[doktrin]

It's practically impossible to be a web developer without knowing anything about databases.

Most of the topics the parent listed are significantly more complex than "knowing about databases", which implies that "database design" in this context isn't simply drawing an ER diagram and correctly identifying the purpose of a foreign key.

Given that the other topics listed include sockets, parallelism & map reduce, my guess is that "database design" might in this case mean distributed database concepts and/or sharding.