Translating SQL to RA expression is the second step in Query Processing Pipeline Input: Logical Query Plan - expression in Extended Relational Algebra; Output: Optimized Logical Query Plan - also in Relational Algebra; Union, Intersection, Difference. Relational Algebra is not a full-blown SQL language, but rather a way to gain theoretical understanding of relational processing. In practice, SQL is the query language that is used in most commercial RDBMSs. Relational Algebra on Bags A bag(or multiset) is like a set, but an element may appear more than once. Translating SQL to Relational Algebra. Example: {1,2,1,3} is a bag. grouping mechanism can be added to relational algebra without increasing its computa-tional power. Translating SQL Queries into Relational Algebra . As such it shouldn't make references to physical entities such as tables, records and fields; it should make references to abstract constructs such as relations, tuples and attributes. Not relational algebra, but in SQL, it looks like this: SELECT playerName, count() FROM Competition GROUP BY playerName HAVING COUNT() > 1 – cliffordheath Mar 28 '14 at 23:49 1 There's no single "relational algebra". A (general or theta θ) join of R and S is the expression R join-condition S. I To process a query, a DBMS translates SQL into a notation similar to relational algebra. Relational Algebra is a procedural query language which takes relations as an input and returns relation as an output. 30 Why Bags? Translation is … SQL, the most important query language for relational databases, is I Relational algebra eases the task of reasoning about queries. Introduction The concept of grouping in relational algebra is well-known from its connection to aggregation, and grouping constructs such as group by 3,4 have been defined in order to incorporate the ideas into relational languages. Relational Algebra (Reference: Chapter 4 of Ramakrishnan & Gehrke) IT2002 (Semester 1, 2004/5): Relational Algebra 68 Example Database Movies title director myear rating Fargo Coen 1996 8.2 Raising Arizona Coen 1987 7.6 Spiderman Raimi 2002 7.4 Wonder Boys Hanson 2000 7.6 Actors actor ayear Cage 1964 Hanks 1956 Maguire 1975 However, when a WHERE clause is used rather than a HAVING clause, the resulting row set (tuples) can be quite different. Basics of Relational model: Relational Model. An SQL query is first translated into an equivalent extended relational algebra expression—represented as a query tree data structure—that is then optimized. T. M. Murali August 30, 2010 CS4604: SQL and Relational Algebra Based on what the HAVING clause does (filter), it seems that it would be expressed in relational algebra as a predicate as well. 1. Example: {1,2,3} is also a bag that happens to be a set. It is a convenience operation because it is done so much. I Operations in relational algebra have counterparts in SQL. “Pure” relational algebra uses mathematical notation with Greek letters I will cover it using SQL syntax; that is in this unit I will cover relational algebra, but it will look like SQL • and will be really valid SQL Pure relational algebra is used in research, scientific papers, and some textbooks (mainly because of inertia) There are some basic operators which can be applied on relations to produce required results which we will discuss one by one. 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