Database System Concepts, 6 th Ed. ©Silberschatz, Korth and Sudarshan See for conditions on re-usewww.db-book.com Chapter 2: Intro to Relational Model

©Silberschatz, Korth and Sudarshan2.2Database System Concepts - 6 th Edition Example of a Relation attributes (or columns) tuples (or rows)

©Silberschatz, Korth and Sudarshan2.3Database System Concepts - 6 th Edition Attribute Types The set of allowed values for each attribute is called the domain of the attribute Eg., Domain of name attribute is a set of all possible names. Attribute values are (normally) required to be atomic; that is, indivisible Eg., a attribute phone number is atomic if it stores a single phone number The special value null is a member of every domain The null value causes complications in the definition of many operations

©Silberschatz, Korth and Sudarshan2.4Database System Concepts - 6 th Edition Relation Schema and Instance Relation schema Defines the logical design of the relation Example: student(sid, name) Relation instance Snapshot of the data in the relation at a given instant in time Example: sidname John Mary

©Silberschatz, Korth and Sudarshan2.5Database System Concepts - 6 th Edition Relations are Unordered Order of tuples is irrelevant (tuples may be stored in an arbitrary order) Example: instructor relation with unordered tuples

©Silberschatz, Korth and Sudarshan2.6Database System Concepts - 6 th Edition Database A database consists of multiple relations Information about an enterprise is broken up into parts instructor student advisor Bad design: univ (instructor -ID, name, dept_name, salary, student_Id,..) results in repetition of information (e.g., two students have the same instructor) the need for null values (e.g., represent an student with no advisor) Normalization theory (Chapter 7) deals with how to design “good” relational schemas

©Silberschatz, Korth and Sudarshan2.7Database System Concepts - 6 th Edition Keys Let K  R K is a superkey of R if values for K are sufficient to identify a unique tuple of each possible relation r(R) Example: {ID} and {ID,name} are both superkeys of instructor. Superkey K is a candidate key if K is minimal Example: {ID} is a candidate key for Instructor One of the candidate keys is selected to be the primary key. Foreign key constraint: Value in one relation must appear in another Referencing relation Referenced relation

©Silberschatz, Korth and Sudarshan2.8Database System Concepts - 6 th Edition Schema Diagram for University Database

©Silberschatz, Korth and Sudarshan2.9Database System Concepts - 6 th Edition Relational Query Languages Procedural vs. non-procedural, or declarative In a procedural language, the user instructs the system to perform a sequence of instructions on the database to compute the desired result In a nonprocedural language, the user describes the desired information without giving a specific procedure for obtaining the information Pure query languages: Relational algebra – procedural Tuple relational calculus - nonprocedural Domain relational calculus - nonprocedural Relational operators

©Silberschatz, Korth and Sudarshan2.10Database System Concepts - 6 th Edition Selection of tuples Relation r Select tuples with A=B and D > 5 σ A=B and D > 5 (r)

©Silberschatz, Korth and Sudarshan2.11Database System Concepts - 6 th Edition Selection of Columns (Attributes) Relation r: Select A and C Projection Π A, C (r)

©Silberschatz, Korth and Sudarshan2.12Database System Concepts - 6 th Edition Joining two relations – Cartesian Product Relations r, s: r x s:

©Silberschatz, Korth and Sudarshan2.13Database System Concepts - 6 th Edition Union of two relations Relations r, s: r  s:

©Silberschatz, Korth and Sudarshan2.14Database System Concepts - 6 th Edition Set difference of two relations Relations r, s: r – s:

©Silberschatz, Korth and Sudarshan2.15Database System Concepts - 6 th Edition Set Intersection of two relations Relation r, s: r  s

©Silberschatz, Korth and Sudarshan2.16Database System Concepts - 6 th Edition Natural Join Example Relations r, s: Natural Join r s

©Silberschatz, Korth and Sudarshan2.17Database System Concepts - 6 th Edition Figure in-2.1

Database System Concepts, 6 th Ed. ©Silberschatz, Korth and Sudarshan See for conditions on re-usewww.db-book.com End of Chapter 2