We investigate classes of SQL queries which are syntactically correct, but certainly not intended, no matter for which task the query was written. For instance, queries that are contradictory, i.e. always return the empty set, are obviously not intended. However, current database management systems execute such queries without any warning. In this paper, we give an extensive list of conditions that are strong indications of semantic errors. Of course, questions like the satisfiability are in general undecidable, but a significant subset of SQL queries can actually be checked. We believe that future database management systems will perform such checks and that the generated warnings will help to develop code with fewer bugs in less time.