The question shows that you have a hard time to understand the complexity of it. Maybe the reason is that you are an experienced developer who deals with all the complexity quite naturally and automatically.
So why is this task complex?
Well, what happens if there are no numbers in the table to begin with? Is the sum of no numbers a zero or is it supposed to be some error?
And are all the numbers supposed to be natural numbers or can there be e.g. irrational and complex numbers be in there? If so, how much precision do we need when summing them up - and how important is performance?
Also, what if there are not only numbers but other things (dates, text, ...) by accident?
And in a real world scenario, what happens when numbers are added to the table during the calculation? Should they be considered or ignored - should the table be locked somehow?
The problem with these questions is not that they are necessarily hard to answer (often they are) but that people don't even know, that they need to be asked in the beginning.
Many developers are so used to it, they often don't understand that they are doing things that are difficult for normal people.
In the same way mathematicians find it easy to do basic algebra for some more difficult task, it's just part of there toolkit - whereas most people don't even understand this basic part from the beginning, let alone more complex problems.