In this paper, we deal with examining the expressive power of the SQL language, mostly in regards to the inability to express recursive queries in the core SQL language which encompasses the standard from 1992. Expressiveness boundaries are set using the feature of locality. We provide a draft of the proof of the locality of queries by using the extension of relational algebra via aggregates, aggregate logic and counting logic. We deal with an important addition to the SQL language in the standard from 1999 recursion, as well as consider the limits related to defining recursive queries in that standard. In the first chapter, we define terms related to the relational database model and relational algebra operators. We showcase the effect of the defined operators by giving a practical example. In the second chapter, we describe the effect of core SQL operators while using the database examples from the first chapter. We also provide a short overview of the history of SQL with regards to language standardization. In the last part of the chapter we deal with recursion, the motivation for introducing recursion and give practical examples of recursive queries. In the third and final chapter, we introduce the extension of relational algebra via aggregate functions, grouping and arithmetic operations, denoted by ALG\(_\text{aggr}\). We introduce the notion of locality and use it to state the results of the expressive power of SQL. Furthermore, we provide a sketch of the proof of an important result (theorem 3.3.8), while also introducing two query languages based on logic - \(\mathcal{L}_\text{aggr}\) and \(\mathcal{L}_\text{C}\). At the end of the chapter, we showcase the limitations related to defining recursive queries and also deal with query features such as monotonicity, linearity, the existence and uniqueness of a fixed point, etc.