NoSQL is a new approach to data storage and manipulation. The aim of this paper is to gain more insight into NoSQL databases, as we are still in the early stages of understanding when to use them and how to use them in an appropriate way. In this submission descriptions of selected NoSQL databases are presented. Each of the databases is analysed with primary focus on its data model, data access, architecture and practical usage in real applications. Furthemore, the NoSQL databases are compared in fields of data references. The relational databases offer foreign keys, whereas NoSQL databases provide us with limited references. An intermediate model between graph theory and relational algebra which can address the problem should be created. Finally, the proposal of a new approach to the problem of inconsistent references in Big Data storage systems is introduced.