An invariant is something that must always be true. The most basic example of this is a not-null foreign key, where a value in one table refers to a row in another table and that row in the other table must always exist.

> Are you really doing inserts exclusively based on the data of another table? You never take user input from a website? You never need to just get a list of data according to a query with some filtering?

None of these go against what GP said in any way.