1. A computer-implemented method comprising: providing a database organized according to a relational model; providing a database engine in communication with the database through a language describing the relational model; providing an application comprising an entity-relationship model (ERM), the ERM including a first entity, a second entity, and a relationship between the first entity and the second entity; and causing a query engine of the application to communicate a query to the database engine utilizing a language extension comprising, a first structured entity type including a first key and indicating the first entity, a second structured entity type including a second key and indicating the second entity, and a structured association type reflecting the relationship and including a first parameter clause specifying cardinality information, and including a second parameter clause specifying a combination of alternative key elements in a nested structure comprising an alias name of one alternative key element in a substructure of the second structured entity; and causing the database engine to return a query result to the query engine based upon the language extension.