Outline Structure of Relational Databases Database Schema Keys Schema Diagrams Relational Query Languages The Relational Algebra Database System Concepts-7th Edition 2.2 @Silberschatz,Korth and Sudarshan
Database System Concepts - 7 2.2 ©Silberschatz, Korth and Sudarshan th Edition Outline ▪ Structure of Relational Databases ▪ Database Schema ▪ Keys ▪ Schema Diagrams ▪ Relational Query Languages ▪ The Relational Algebra
Example of a Instructor Relation attributes (or columns) ID name dept name salary 10101 Srinivasan Comp.Sci. 65000 tuples 12121 Wu Finance 90000 (or rows) 15151 Mozart Music 40000 22222 Einstein Physics 95000 32343 El Said History 60000 33456 Gold Physics 87000 45565 Katz Comp.Sci. 75000 58583 Califieri History 62000 76543 Singh Finance 80000 76766 Crick Biology 72000 83821 Brandt Comp.Sci. 92000 98345 Kim Elec.Eng. 80000 Database System Concepts-7th Edition 2.3 @Silberschatz,Korth and Sudarshan
Database System Concepts - 7 2.3 ©Silberschatz, Korth and Sudarshan th Edition Example of a Instructor Relation attributes (or columns) tuples (or rows)
Relation Schema and Instance A1,A2,...,An are attributes R=(A,A2,...,A is a relation schema Example: instructor =(ID,name,dept name,salary) A relation instance r defined over schema R is denoted by r(R) The current values a relation are specified by a table An element t of relation ris called a tuple and is represented by a row in a table Database System Concepts-7th Edition 2.4 @Silberschatz,Korth and Sudarshan
Database System Concepts - 7 2.4 ©Silberschatz, Korth and Sudarshan th Edition Relation Schema and Instance ▪ A1 , A2 , …, An are attributes ▪ R = (A1 , A2 , …, An ) is a relation schema Example: instructor = (ID, name, dept_name, salary) ▪ A relation instance r defined over schema R is denoted by r (R). ▪ The current values a relation are specified by a table ▪ An element t of relation r is called a tuple and is represented by a row in a table
Attributes 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 special value null is a member of every domain.Indicated that the value is“unknown” The null value causes complications in the definition of many operations Database System Concepts-7th Edition 2.5 ©Silberscha乜，Korth and Sudarshan
Database System Concepts - 7 2.5 ©Silberschatz, Korth and Sudarshan th Edition Attributes ▪ 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 special value null is a member of every domain. Indicated that the value is “unknown” ▪ The null value causes complications in the definition of many operations
Relations are Unordered Order of tuples is irrelevant(tuples may be stored in an arbitrary order) Example:instructor relation with unordered tuples ID name dept name salary 22222 Einstein Physics 95000 12121 Wu Finance 90000 32343 El Said History 60000 45565 Katz Comp.Sci. 75000 98345 Kim Elec.Eng. 80000 76766 Crick Biology 72000 10101 Srinivasan Comp.Sci. 65000 58583 Califieri History 62000 83821 Brandt Comp.Sci. 92000 15151 Mozart Music 40000 33456 Gold Physics 87000 76543 Singh Finance 80000 Database System Concepts-7th Edition 2.6 @Silberschatz,Korth and Sudarshan
Database System Concepts - 7 2.6 ©Silberschatz, Korth and Sudarshan th Edition Relations are Unordered ▪ Order of tuples is irrelevant (tuples may be stored in an arbitrary order) ▪ Example: instructor relation with unordered tuples