Chapter 3: Relational Model Structure of Relational Databases Relational Algebra Tuple Relational Calculus Domain Relational Calculus Extended Relational-Algebra-Operations Modification of the Database Views Example of a Relation [IMG]file:///C:\Users\MANGESH\AppData\Local\Temp\msohtmlclip1\01\clip_image002.jpg[/IMG] Attribute Types Each attribute of a relation has a name The set of allowed values for each attribute is called the domain of the attribute Attribute values are (normally) required to be atomic, that is, indivisible E.g. multivalued attribute values are not atomic E.g. composite attribute values are not atomic The special value null is a member of every domain The null value causes complications in the definition of many operations we shall ignore the effect of null values in our main presentation and consider their effect later Relation Schema A1, A2, …, An are attributes R = (A1, A2, …, An ) is a relation schema E.g. Customer-schema = (customer-name, customer-street, customer-city) r(R) is a relation on the relation schema R E.g. customer (Customer-schema) Relation Instance The current values (relation instance) of a relation are specified by a table An element t of r is a tuple, represented by a row in a table Database n A database consists of multiple relations n Information about an enterprise is broken up into parts, with each relation storing one part of the information E.g.: account : stores information about accounts depositor : stores information about which customer owns which account customer : stores information about customers n Storing all information as a single relation such as bank(account-number, balance, customer-name, ..) results in H repetition of information (e.g. two customers own an account) H the need for null values (e.g. represent a customer without an account)
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