Relational Database Model. III. Introduction to the Relational Database Model. Relational Database Model. Relational Terminology.

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1 III. Introduction to the Relational Database Model Relational Database Model In 1970, E. F. Codd published A Relational Model of Data for Large Shared Data Banks in CACM. In the early 1980s, commercially viable relational database management systems became available. 2 Relational Database Model While relational database was very tempting in concept in the 1970s, it was not easily applicable in a real-world environment for reasons related to performance. The earlier hierarchical and network database management systems were just coming onto the commercial scene and were the focus of intense marketing efforts by the software and hardware vendors. The Relational Database Concept Data appears to be stored in what we have been referring to as simple, linear files. Relational databases are based on mathematics. Relational algebra A relational database is a collection of relations that, as a group, contain the data that describes a particular business environment. 3 4 Relational Terminology Relation logical representation of an underlying file. Also called a table. Row = record (files) = tuple (relation) Column = field (files) = attribute (relation) Relation schema names a relation and lists its attributes (including keys and other properties) 5 Dependencies Functional dependence: Value of one or more attributes determines the value of one or more other attributes Determinant: Attribute whose value determines another Dependent: Attribute whose value is determined by the other attribute Full functional dependence: Entire collection of attributes in the determinant is necessary for the relationship 6 1

2 Primary Key A relation always has a unique primary key A primary key (also called the key ) is an attribute or a group of attributes whose values are unique throughout all of the rows of the relation All attributes are functionally dependent on the key Used to: Ensure that each row in a table is uniquely identifiable Establish relationships among tables and to ensure the integrity of the data Primary Key Specified in the relation schema The number of attributes involved in the primary key is always the minimum number of attributes that provide the uniqueness quality In the worst case, all of the relation s attributes combined could serve as the primary key 7 8 Candidate Key If a relation has more than one attribute or minimum group of attributes that represents a way of uniquely identifying the entities, then they are each called a candidate key When there is more than one candidate key, one of them must be chosen to be the primary key of the relation Alternate key is a candidate key that was not chosen to be the primary key of the relation 9 Foreign Key An attribute or group of attributes that serves as the primary key of one relation and also appears in another relation (foreign key in this relation). 10 Foreign Key An attribute or group of attributes that serves as the primary key of one relation and also appears in another relation (foreign key in the second relation). Crucial in relational database, because the foreign key is the mechanism that ties relations together to represent unary, binary, and ternary relationships. Foreign key attribute must have same domain of values (i.e., type) as primary key attribute in other relation. Keys and Integrity Constraints Entity integrity: Condition in which each row in the table has its own unique identity All of the values in the primary key must be unique No key attribute in the primary key can contain a null Referential integrity: Every reference to an entity instance by another entity instance is valid

3 Integrity Rules Integrity Rules Referential Integrity Description Entity Integrity Requirement Purpose Description All primary key entries are unique, and no part of a primary key may be null Each row will have a unique identity, and foreign key values can properly reference primary key values Requirement Purpose A foreign key may have either a null entry or a entry that matches a primary key value in a table to which it is related It is possible for an attribute not to have a corresponding value but it is impossible to have an invalid entry Example No invoice can have a duplicate number, nor it can be null It is impossible to delete row in a table whose primary keys has mandatory matching foreign key values in another table 13 Example It is impossible to have invalid sales representative number 14 Relational Database Model Example of a Simple Relational Database Relations used to implement building blocks from the data model: Entity sets Including dependent entity sets Attributes Including composite, multi-valued, derived Relationships Binary, unary, ternary 1-1, 1-M, M-M Constraints Specialization/generalization hierarchies Binary Relationships One-to-One One-to-Many Binary Relationship Customer One-to-Many Many-to-Many The Number foreign key in the CUSTOMER relation effectively establishes the one-to-many relationship between salespersons and customers

4 General Hardware Co. One-to-One Binary Relationship Commission Year Number Name Percentage Of Hire 137 Baker Adams Dickens Carlyle SALESPERSON relation. Customer Customer Number Name Number HQ City 0121 Main St. Hardware 137 New York 0839 Jane s Stores 186 Chicago 0933 ABC Home Stores 137 Los Angeles 1047 Acme Hardware Store 137 Los Angeles 1525 Fred s Tool Stores 361 Atlanta 1700 XYZ Stores 361 Washington 1826 City Hardware 137 New York 2198 Western Hardware 204 New York 2267 Central Stores 186 New York CUSTOMER relation General Hardware Co. including OFFICE Many-to-Many Relationship Commission Year Office Number Name Percentage Of Hire Number 137 Baker Adams Dickens Carlyle SALESPERSON relation. Office Number Telephone Size (sq. ft.) OFFICE relation. Can SALESPERSON and OFFICE be combined into one relation? Could foreign key be in OFFICE instead? Unacceptable: Many-to-Many Number Name Commission Percentage Year Of Hire Product and Quantity Pairs 137 Baker (19440, 473) (24013, 170) (26722, 688) 186 Adams (16386, 1745) (19440, 2529) (21765, 1962) (24013, 3071) 204 Dickens (21765, 809) (26722, 734) 361 Carlyle (16386, 3729) (21765, 3110) (26722, 2738) 23 Many-to-Many Relationship Has its own relation in the database Can have its own attributes Intersection data It is a kind of entity -- an associative entity (or composite or bridge) 24 4

5 SALES Relation (without intersection data) Number Product Number Many-to-Many Relationship Commission Year Office Number Name Percentage Of Hire Number 137 Baker Adams Dickens Carlyle SALESPERSON relation. Product Number Number Quantity SALES relation. Product Product Number Name Unit Price Wrench Hammer Drill Saw Pliers PRODUCT relation. Product 26 General Hardware Co. including OFFICE Data Retrieval from a Relational Database Commission Year Office Number Name Percentage Of Hire Number 137 Baker Adams Dickens Carlyle (a) SALESPERSON relation. Customer Customer Number Name Number HQ City 0121 Main St. Hardware 137 New York 0839 Jane s Stores 186 Chicago 0933 ABC Home Stores 137 Los Angeles 1047 Acme Hardware Store 137 Los Angeles 1525 Fred s Tool Stores 361 Atlanta 1700 XYZ Stores 361 Washington 1826 City Hardware 137 New York 2198 Western Hardware 204 New York 2267 Central Stores 186 New York (b) CUSTOMER relation. Customer Employee Employee Title Number Number Name Smith Co-Owner Garcia Co-Owner Have the ability to accept high level data Chen VP Sales Levy Sales Manager retrieval commands Morton President Baker Sales Manager Smith President Process the commands against the Jones VP Sales Garcia VP Personnel database s relations Kaplan Senior Accountant (c) CUSTOMER EMPLOYEE relation. and return the desired data. Product Product Unit Number Name Price Wrench Based on relational algebra Hammer Drill Saw Pliers (d) PRODUCT relation Relational DBMS Relational Algebra The formal basis for operations on the relational data model Underlying basis for both SQL and QBE An algebra is a collection of operations on some domain Relational algebra is a collection of operators operands and results are relations (closure) significant operators projection and selection remove parts of a relation set operators, union, intersection and difference joins and products combine the tuples of two relations 29 Select Operator Retrieves a horizontal slice of the relation. The result of a relational operation will always be a relation. applied to R produces a subset of R Applies a selection criterion to a relation Select all rows from the SALESPERSON relation in which Number = 204. Select all rows from the MOVIE relation that are longer than 100 minutes and genre is action comedy 30 5

6 Select Project Operator Retrieves a vertical slice of the relation. Used to produce a relation with fewer attributes (columns) called projection because it reduces the dimension of the relation e.g., removing a column from a 3-dimensional relation produces a 2-dimensional relation Examples List (project) the Title of every Movie List the Number and Name over the SALESPERSON relation. May have fewer rows than the original relation Project Set Operations Union: R S = { t t R or t S} R and S must have the same number and types of attributes No duplicate tuples Consider union of tables Rental (accountid, videoid, daterented, datedue, cost) PreviousRental (accountid, videoid, daterented, datereturned, cost) Rental PreviousRental Intersection and Difference are similarly based on normal set operations Union Intersect

7 Difference Cartesian Product Cartesian product of two relations R and S creates a new relation one attribute for each attribute in R and one for each attribute in S one tuple for each combination of tuple in R and tuple in S R * S tuples if R has m tuples and S has n tuples, R S has m*n tuples Just like Cartesian product of any set Product Data Integration The relational algebra Join operation Based on Cartesian product R join S on A has those tuples of R S where R.A = S.A Each tuple from R is joined to all tuples of S that have the same value for attribute A Example Every combination of and Customer where the Number fields match How would I find the name of the salesperson for City Hardware? Terminology Natural JOIN Cartesian Product - comparing every possible combination of two sets, or two relations Natural join - Links tables by selecting only the rows with common values in their common attributes Join columns: Common columns Product Select Project Equijoin - Links tables on the basis of an equality condition that compares specified columns of each table Theta join uses a condition other than equality

8 Types of Joins Left and Right JOINs Inner join: Only returns matched records from the tables that are being joined Outer join: Matched pairs are retained and unmatched values in the other table are left null Left outer join: Yields all of the rows in the first table, including those that do not have a matching value in the second table Right outer join: Yields all of the rows in the second table, including those that do not have matching values in the first table Relational Expressions Data Dictionary and the System Catalog Select customer 1826, project name and HQ city Customer Name, HQ City ( Customer Number=1826 (Customer)) name for City Hardware Name (( Customer Name= City Hardware (Customer)) Number ) Name ( Customer Name= City Hardware ( Number Customer)) What is the order of evaluation? What is the best order of evaluation? Query optimization 45 Data dictionary: Description of all tables in the database created by the user and designer System catalog: System data dictionary that describes all objects within the database Homonyms and synonyms must be avoided to lessen confusion Homonym: Same name is used to label different attributes Synonym: Different names are used to describe the same attribute 46 Index Orderly arrangement to logically access rows in a table Index key: Index s reference point that leads to data location identified by the key Unique index: Index key can have only one pointer value associated with it Each index is associated with only one table 47 8