Database black box testing, how to.

So i know the fundamentals of black box testing and what it is, in short it is to examine the input to the system and checking the output of the system, without bothering the system (code/underlining, how the system generates the output)

My question however is how I go about black box testing a database? Where do i even start? With a website for example, its easy, you know what a button is meant to do, (the input) and you explain the results of what actually happened (the output).

But i am struggling to get my head around where I start with a database?

Say for instance i have four tables within a database, with data sitting inside of each one, how do i "Test" anything?

I have browsed for similar questions although i haven't found what it is Im looking for.