relational calculus

whereas the relational algebra defines a set of operations for the relational model, the relational calculus provides a higher-level declarative language for specifying relational queries. in a relational calculus expression, there is no order of operations to specify how to retrieve the query result--only what information the result should contain. this is the main distinguishing feature between relational algebra and relational calculus. the relational calculus is important because it has a firm basis in mathematical logic and because the standard query language (SQL) for RDBMS has some of its foundations in a variation of relational calculus known as the tuple relational calculus.
[cite:@elmasri_db_2015 chapter 8 the relational algebra and relational calculus]