11.5. Database Access Languages

There are two types of database access languages:

1) data description language - a high-level language designed to describe the logical structure of the data;

2) data manipulation language - a set of structures that ensure the performance of basic operations on working with data: input, modification and selection of data by queries.

The most common access languages ​​are two standardized languages:

1) QBE (Query by Example) is a pattern-based query language characterized by the properties of a data manipulation language;

2) SQL (Structured Query Language) is a structured query language consisting of the properties of languages ​​of both types.

The QBE language was developed on the basis of relational calculus with variable domains. It helps to form complex queries to the database using the filling of the request form with the proposed database management system. Any of the relational DBMS has its own version of the QBE language. The advantages of this method of specifying queries to the database are:

1) high visibility;

2) no need to specify the algorithm for the operation.

A structured SQL query language appeared on the basis of relational calculus with variable tuples. Several standards have been developed for this language, the most famous of which are SQL-89 and SQL-92. The SQL language is used to perform operations on tables and data that are contained in these tables, and some related operations. It is not used as a separate language and most often serves as part of the embedded programming language of the DBMS (for example, FoxPro DBMS Visual FoxPro, ObjectPAL Paradox DBMS, Visual Basic for Applications DBMS Access).

The SQL language is focused only on data access, so it is referred to as software development tools and is called embedded. There are two main methods of using embedded SQL:

1) static - characterized by the fact that the text of the program placed calls to functions of the SQL language, rigidly included in the executable module after compilation. Changes in called functions can be made at the level of individual call parameters using programming language variables;

2) dynamic - differs in dynamic construction of calls to SQL functions and interpretation of these calls during program execution. It is most often used in cases when the type of SQL call in the application is not known in advance, and it is built up in dialogue with the user.