If the emperor of China asked you to make sure that the wall is intact, you would need to send riders to every part of it and have them inspect it for you. For a data set, these riders are your database queries. The catch is that they are very stupid: They are only able to answer a single question that you ask them when you send them on their journey to inspect the wall. If you asked them to tell you about the state of the bricks, they would only tell you whether they saw any broken or damaged bricks. They wouldn't tell you if parts of the wall were missing. They wouldn't tell you if there were tunnels under the wall. And they wouldn't tell you if the wall was too thin, not high enough or in other ways poorly built somewhere along the border. In order to confidently say that the wall is fine, you would need to think of every conceivable question necessary to make sure that there are no parts of the wall that would allow enemies to get in.