Abstract Expressive query languages are gaining relevance in knowledge representation (KR), and new reasoning problems come to the fore. Especially query containment is interesting in this context. The problem is known to be decidable for many expressive query languages, but exact complexities are often missing. We introduce a new query language, guarded queries (GQ), which generalizes most known languages where query containment is decidable. GQs can be nested (more expressive), or restricted to linear recursion (less expressive). Our comprehensive analysis of the computational properties and expressiveness of (linear/nested) GQs also yields insights on many previous languages.