2.2.2 Classification of relational data languages

Relational data language--
           |--Relational algebra language such as ISBL |--Relational

           calculus language--
                         |--Tuple relational calculus language such as APLHA,QUEL
                         |--Domain relational calculus language such as QBE

           |--Has relational algebra and relations Languages ​​with dual characteristics of calculus such as SQL


's early relational manipulation capabilities are usually expressed in algebraic or logical ways:
relational algebra (relational algebra) is used to express query requirements by operating on relations.
Relation calculus uses predicates to express query requirements. Relational calculus can be divided into tuple relational calculus and domain relational calculus according to whether the basic object of the predicate argument is a tuple variable or a domain variable.


SQL is a Structured Query Language between relational algebra and relational calculus. SQL not only has rich query functions, but also has data definition and data control functions. It is a relational data language that integrates query, DDL, DML and DCL. It fully embodies the characteristics and advantages of relational data language, and is the markup language of relational database.


    The relational language is a highly non-procedural language, and the user does not need to request the DBA to establish a special access path for it. The selection of the access path is completed by the optimization mechanism of the RDBMS. For example, in a relationship that stores millions of records to find one or some records that meet the conditions, in principle, there can be multiple search methods. For example, this relationship can be scanned sequentially, or it can be searched through a certain index. Different search paths (or access paths) have different efficiencies. Some may complete a query very quickly, while others may be very slow. The query optimization method is researched and developed in RDBMS, the system can automatically select the optimal access path and improve the query efficiency.