Chapter 2 introduces the relational model of data, covering basic concepts such as the structure of relational databases, database schemas, keys, schema diagrams, relational query languages, and relational operations. | Chapter 2: Relational Model Chapter 2: Relational Model Structure of Relational Databases Fundamental Relational-Algebra-Operations Additional Relational-Algebra-Operations Extended Relational-Algebra-Operations Null Values Modification of the Database Example of a Relation 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 . the value of an attribute can be an account number, but cannot be a set of account numbers Domain is said to be atomic if all its members are 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 Formally, given domains D1, D2, . Dn a relation r is a subset of D1 x D2 x x Dn Thus, a relation is a set of n-tuples (a1, a2, , an) where each ai Di Schema of a relation consists of attribute definitions name type/domain integrity constraints 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 Order of tuples is irrelevant (tuples may be stored in an arbitrary order) Jones Smith Curry Lindsay customer_name Main North North Park customer_street Harrison Rye Rye Pittsfield customer_city customer attributes (or columns) tuples (or rows) Database A database consists of multiple relations Information about an enterprise is broken up into parts, with each relation storing one part of the information . account : information about accounts depositor : which customer owns which account customer : information about customers The customer Relation The depositor Relation Why Split Information Across Relations? Storing all information as a single relation such as bank(account_number, . | Chapter 2: Relational Model Chapter 2: Relational Model Structure of Relational Databases Fundamental Relational-Algebra-Operations Additional Relational-Algebra-Operations Extended Relational-Algebra-Operations Null Values Modification of the Database Example of a Relation 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 . the value of an attribute can be an account number, but cannot be a set of account numbers Domain is said to be atomic if all its members are 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 Formally, given domains D1, D2, . Dn a relation r is a subset of D1 x D2 x x Dn Thus, a relation is a set
