Basics of Relational model: Relational Model. grouping mechanism can be added to relational algebra without increasing its computa-tional power. 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. Relational Algebra on Bags A bag(or multiset) is like a set, but an element may appear more than once. Translating SQL Queries into Relational Algebra . Translating SQL to Relational Algebra. 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. This is a derived operation, i.e., it is based on the basic operations of the relational algebra. It is a convenience operation because it is done so much. Example: {1,2,1,3} is a bag. “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) However, when a WHERE clause is used rather than a HAVING clause, the resulting row set (tuples) can be quite different. 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 Relational Algebra is a procedural query language which takes relations as an input and returns relation as an output. 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". I To process a query, a DBMS translates SQL into a notation similar to relational algebra. T. M. Murali August 30, 2010 CS4604: SQL and Relational Algebra 1. SQL, the most important query language for relational databases, is An SQL query is first translated into an equivalent extended relational algebra expression—represented as a query tree data structure—that is then optimized. Example: {1,2,3} is also a bag that happens to be a set. I Operations in relational algebra have counterparts in SQL. There are some basic operators which can be applied on relations to produce required results which we will discuss one by one. Based on what the HAVING clause does (filter), it seems that it would be expressed in relational algebra as a predicate as well. In practice, SQL is the query language that is used in most commercial RDBMSs. Translation is … A (general or theta θ) join of R and S is the expression R join-condition S. I Relational algebra eases the task of reasoning about queries. Relational Algebra is not a full-blown SQL language, but rather a way to gain theoretical understanding of relational processing. • Aggregate operation in relational algebra E is any relational-algebra expression –G1, G2 …, Gn is a list of attributes on which to group (can be empty) –Each F i is an aggregate function –Each A i is an attribute name • Note: Some books/articles use γ instead of (Calligraphic G), , , ( ), ( … 30 Why Bags?