This paper, the first step to connect relational databases with systems consequence (Kent: "System Consequence" 2009), is concerned with the semantics of relational databases. It aims to to study system consequence in the logical/semantic system of relational databases. The paper, which was inspired by and which extends a recent set of papers on the theory of relational database systems (Spivak: "Functorial Data Migration" 2012), is linked with work on the Information Flow Framework (IFF) [http://suo.ieee.org/IFF/] connected with the ontology standards effort (SUO), since relational databases naturally embed into first order logic. The database semantics discussed here is concerned with the conceptual level of database architecture. We offer both an intuitive and technical discussion. Corresponding to the notions of primary and foreign keys, relational database semantics takes two forms: a distinguished form where entities are distinguished from relations, and a unified form where relations and entities coincide. The distinguished form corresponds to the theory presented in (Spivak: "Simplicial databases" 2009)[arXiv:0904.2012]. The unified form, a special case of the distinguished form, corresponds to the theory presented in (Spivak: "Functorial Data Migration" 2012). A later paper will discuss various formalisms of relational databases, such as relational algebra and first order logic, and will complete the description of the relational database logical environment.