I To process a query, a DBMS translates SQL into a notation similar to relational algebra. Translating SQL to Relational Algebra. It is a convenience operation because it is done so much. 1. In practice, SQL is the query language that is used in most commercial RDBMSs. Translating SQL Queries into Relational Algebra . Example: {1,2,3} is also a bag that happens to be a set. • 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), , , ( ), ( … 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". “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) Translation is … Relational Algebra on Bags A bag(or multiset) is like a set, but an element may appear more than once. However, when a WHERE clause is used rather than a HAVING clause, the resulting row set (tuples) can be quite different. A (general or theta θ) join of R and S is the expression R join-condition S. This is a derived operation, i.e., it is based on the basic operations of the relational algebra. grouping mechanism can be added to relational algebra without increasing its computa-tional power. SQL, the most important query language for relational databases, is Example: {1,2,1,3} is a bag. 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. I Operations in relational algebra have counterparts in SQL. 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. Basics of Relational model: Relational Model. I Relational algebra eases the task of reasoning about queries. Relational Algebra is a procedural query language which takes relations as an input and returns relation as an output. 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. 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. An SQL query is first translated into an equivalent extended relational algebra expression—represented as a query tree data structure—that is then optimized. 30 Why Bags? 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