Can we rely on SQL?

It is important for any data language that it enables many people to derive correct information from a databases in a simple, effective way with predictable performance. In an analysis, is shown that SQL cannot fulfill these essential preconditions. It is caused by insufficient structural semantics of the relational model and the related operations in SQL. This follows from a comparison with results obtained from application of the semantic Xplain data language. The consequences of these shortcomings are an extreme performance degradation and a growing uncertainty among SQL users