Database Systems: Concepts, Design and Applications

11.4. Query Decomposition

The query decomposition is the first phase of query processing whose aims are to transform a high-level query into a relational algebra query and to check whether that query is syntactically and semantically correct. Thus, a query decomposition phase starts with a high-level query and transforms into a query graph of low-level operations (algebraic expressions), which satisfies the query. In practice, SQL (a relational calculus query) is used as high-level query language, which is used in most commercial RDBMSs. The SQL is then decomposed into query blocks (low-level operations), which form the basic units. The query block contains expressions such as a single SELECT-FROM-WHERE, as well as clause such as GROUP BY and ...