The goal of this thesis is to review modern open-source relational and nonrelational databases and to discover if current trends are converging these two types of databases. The thesis contains systematic description of transferred concepts from relational to nonrelational databases and the other way around. Transferred concepts are also evaluated with benchmark tests. Thesis contains description and evaluation of NewSQL systems and provides opinion whether they contain best of both worlds. We describe relational, nonrelational and NewSQL systems, their weaknesses and strengths. From articles and documentations we extract concepts which lately appeared in PostgreSQL, MariaDB, MySQL, FoundationDB and VoltDB. We describe them in a systematic (similar and short) way and support our claims with examples of program code. We set up virtual environment and developed series of simple benchmark tests in Java for most of transferred concepts. Concepts are evaluated with quantitative tests (speed of operations) and qualitatively (examples of use). At the end there is our analysis of current state of convergence between systems. The main contribution of thesis is a systematic description of transferred concepts compiled in a single document, supported with test results.