In this paper we introduce a syntax and semantics for capturing complex relational behaviour commonly found in natural language system requirements. The syntax is an extension of Behaviour Trees, a modelling notation used for capturing user requirements from natural language in a structured way. The underlying semantics of the extended notation is based on that of relational databases, thereby allowing the expressive power of database queries to be combined with the event- and state-based dynamic behaviour of Behavior Trees. To be a practical method for developing large-scale complex systems, the language is formal and hence supported by simulation and model checking tools.