The return value of a query in the database library is a list of tuples - each list member is a response row, and the tuple is the column values in it.
Why? Because being a list you get them in the same order as the DB returned them, and a tuple - because it's a read-only object (this format actually comes from python's standard db protocol, not from RF's db library itself).
To get the value you're after you have to "unpack" the object; presuming you want to get the first column from the first row, that is the simplest approach:
${db value}= Set Variable ${queryResults[0][0]}
The first index is the row number, the second - the column.
Then you can compare ${db value}
with your expected value.
Keep a couple of things in mind - the access indices of ordered object (lists, tuples) start from 0; and if there is no element at the index you specify - e.g. there was no such row or column in the DB response - this will cause an exception - a fail status in Robotframework.