Relations R and S.
A SQL join clause combines records from two or more tables in a relational database. It creates a set that can be saved as a table or used as it is. A JOIN is a means for combining fields from two tables (or more) by using values common to each. ANSI-standard SQL specifies five types of JOIN: INNER, LEFT OUTER, RIGHT OUTER, FULL OUTER and CROSS. As a special case, a table (base table, view, or joined table) can JOIN to itself in a self-join 1 ), except we use relations as values instead of numbers, and the operations and operators are different. 2  Not used as a query language in actual DBMSs. ( 3 SQL instead.). 4  The inner, lower-level operations of a relational DBMS are, or are similar to, relational algebra operations. 1 Inside a relational DBMS, it is usually much more efficient to calculate a join directly, instead of calculating a cartesian product and then throwing away most of the lines. 2  Note that the same SQL query can be translated to several different relational algebra expressions, which all give the same result. The operation identifies rows that are in the first relation and not in the second relation. The Difference operation is also called the Subtraction operation. PRODUCT. The Product operation can be thought of as a building block that is used to effect a Join operation. Data from two relations are combined. The tables are combined vertically, that is the data from the second table is appended. to the data from the first table. Any row that exists in both relations appears only once in the Union. Operators in relational algebra are not necessarily the same as SQL operators, even if they have the same name. For example, the SELECT statement exists in SQL, and also exists in relational algebra. These two uses of SELECT are not the same. The DBMS must take whatever SQL statements the user types in and translate them into relational algebra operations before applying them to the database. There are two groups of operations: 1  Mathematical set theory based relations: 2  UNION, INTERSECTION, DIFFERENCE, and CARTESIAN PRODUCT. 3  Special database operations: 4  SELECT (not the same as SQL SELECT), PROJECT, and JOIN. Why? 1  Similar to normal algebra (as in. ), 2  except we use relations as values instead of numbers, and the operations and operators are different. 3  Not used as a query language in actual DBMSs. 4  The inner, lower-level operations of a relational DBMS are, or are similar to, relational algebra operations. 5  Some advanced SQL queries requires explicit 6  ... Re 1 Inside a relational DBMS, it is usually much more efficient to calculate a join directly, instead of calculating a cartesian product and then throwing away most of the lines. 2  Note that the same SQL query can be translated to several different relational algebra expressions, which all give the same result. Theta-Join I The theta-join of two relations R and S is the set of tuples in theCartesian product of R and S that satisfy some condition C.RA R ./ C SSQL SELECT *FROM R, SWHERE C; I R ./ C S = ˙ C (R S): T. M. Murali August 30, 2010 CS4604: SQL and Relational Algebra. I Results of operations are also sets of tuples. (Later, we will dene arelational algebra on bags.) I Think of expressions in relational algebra as queries, which construct newrelations from given relations. I Four types of operators: I Remove parts of a single relation: projection and selection. The main application of relational algebra is providing a theoretical foundation for relational databases, particularly query languages for such databases, chief among which is SQL. Whereas the result of a join (or inner join) consists of tuples formed by combining matching tuples in the two operands, an outer join contains those tuples and additionally some tuples formed by extending an unmatched tuple in one of the operands by fill values for each of the attributes of the other operand. 1 Relational algebra is (more) procedural than SQL. (Actually, relational algebra is mathematical expressions.). 1 Inside a relational DBMS, it is usually much more efficient to calculate a join directly, instead of calculating a cartesian product and then throwing away most of the lines. 2  Note that the same SQL query can be translated to several different relational algebra expressions, which all give the same result. The relational algebra uses set union, set difference, and Cartesian product from set theory, but adds additional constraints to these operators. For set union and set difference, the two relations involved must be union-compatible —that is, the two relations must have the same set of attributes. Whereas the result of a join (or inner join) consists of tuples formed by combining matching tuples in the two operands, an outer join contains those tuples and additionally some tuples formed by extending an unmatched tuple in one of the operands by fill values for each of the attributes of the other operand. 1 The inner, lower-level operations of a relational DBMS are, or are similar to, relational algebra operations. 2  We need to know about relational algebra to understand query execution and optimization in a relational DBMS. 3  Some advanced SQL queries requires explicit relational algebra operations, most commonly outer join. 1 Inside a relational DBMS, it is usually much more efficient to calculate a join directly, instead of calculating a cartesian product and then throwing away most of the lines. 2  Note that the same SQL query can be translated to several different relational algebra expressions, which all give the same result.
https://en.wikipedia.org/wiki/Join_(SQL) http://www.databasteknik.se/webbkursen/relalg-lecture/ http://www.tcnj.edu/~letcher/databasestuff/RelationalAlgebra.html http://db.grussell.org/section010.html http://www.databasteknik.se/webbkursen/relalg-lecture/ http://courses.cs.vt.edu/~cs4604/Fall10/lectures/lecture-03-intro-sql-relational-algebra.pdf https://en.wikipedia.org/wiki/Relational_algebra http://www.databasteknik.se/webbkursen/relalg-lecture/ https://en.wikipedia.org/wiki/Relational_algebra http://www.databasteknik.se/webbkursen/relalg-lecture/
types of joins in relational algebra with examples