Ba loại mối quan hệ nhị phân thể hiện trong hình 04,14 có ý nghĩa sau đây: CAN_TEACH liên quan một khóa học để giáo viên hướng dẫn những người có thể dạy cho khóa học; TAUGHT_DURING liên quan đến một học kỳ để giáo viên hướng dẫn giảng dạy một số khóa học trong học kỳ đó, | Basic Relational Algebra Operations The SELECT Operation The PROJECT Operation Sequences of Operations and the RENAME Operation Set Theoretic Operations The JOIN Operation A Complete Set of Relational Algebra Operations The DIVISION Operation In addition to defining the database structure and constraints a data model must include a set of operations to manipulate the data. A basic set of relational model operations constitute the relational algebra. These operations enable the user to specify basic retrieval requests. The result of a retrieval is a new relation which may have been formed from one or more relations. The algebra operations thus produce new relations which can be further manipulated using operations of the same algebra. A sequence of relational algebra operations forms a relational algebra expression whose result will also be a relation. The relational algebra operations are usually divided into two groups. One group includes set operations from mathematical set theory these are applicable because each relation is defined to be a set of tuples. Set operations include UNION INTERSECTION SET DIFFERENCE and CARTESIAN PRODUCT. The other group consists of operations developed specifically for relational databases these include SELECT PROJECT and JOIN among others. The SELECT and PROJECT operations are discussed first because they are the simplest. Then we discuss set operations. Finally we discuss JOIN and other complex operations. The relational database shown in Figure is used for our examples. Some coimnon database requests cannot be performed with the basic relational algebra operations so additional operations are needed to express these requests. Some of these additional operations are described in Section . The SELECT Operation The SELECT operation is used to select a subset of the tuples from a relation that satisfy a selection condition. One can consider the SELECT operation to be a .
