Query Processing

Query Processing

            Upper levels of the data integration problem

     How to construct mappings from sources to a single mediated schema

     How queries posed over the mediated schema are reformulated over the sources

            Basic Steps in Query Processing

            Parsing and translation

            Optimization

            Evaluation

Parsing and translation

Translate the query into its internal form. This is then translated into relational algebra. Parser checks syntax, verifies relations.

Evaluation

The query-execution engine takes a query-evaluation plan, executes that plan, and returns the answers to the query.A relational algebra expression may have many equivalent expressions. Each relational algebra operation can be evaluated using one of several different algorithms. Correspondingly, a relational-algebra expression can be evaluated in many ways. Annotated expression specifying detailed evaluation strategy is called an evaluation-plan.

Query Optimization

Amongst all equivalent evaluation plans choose the one with lowest cost. Cost is estimated using statistical information from the database catalog.

SQL

SQL is Structured Query Language, which is a computer language for storing, manipulating and retrieving data stored in relational database.

SQL is the standard language for Relation Database System. All relational database management systems like MySQL, MS Access, and Oracle, Sybase, Informix, postgres and SQL Server use SQL as standard database language.