Database Systems Data Models & The Relational Model

Transcription

1 Database Systems Data Models & The Relational Model Developing the Database To develop a database you must: Analyze the requirements Logically design the structures Model the data What is a model? Data Models Relatively simple representations, usually graphical, of more complex real world data structures Data models are used to facilitate interaction between designers, developers, and users Data models consist of Entities: Anything about which data is stored Attributes: Characteristics of an entity Relationship: The association between entities 1

2 Using Data Models Database must reflect business rules What are business rules? How do you find business rules? Constraints (validation) are used to ensure data integrity (ex: Age must be a number) Data models are useful in representing business rules Three Levels of Abstraction Physical Level: Describes the files and indexes Lowest level of abstraction How the data are stored Logical (Conceptual) Level: Describes the logical level Describes what data are stored Describes the relationships among the data Database administrator level View Level: How users see the data Highest level of abstraction How the users see the data Can be many views The Relational Model Developed by E.F. Codd (of IBM) in 1970 Made up of several tables that are logically related to each other Links entities together through relationships Entity Relationship Diagram (ERD) Data models used to show the relationship between entities 2

3 Table Characteristics Two Dimensional (Rows and Columns) Each row represents a single occurrence Each column is different Row/Column represents a single data value Each column must have it s own data format String, Number, Date, Boolean, etc Most Important!!! Each table must have an attribute or a combination of attributes that uniquely identifies each row. Keys (Identifiers) Candidate Key: A column or combination of columns containing unique values for each row Primary Key: A specially designated candidate key used to identify each row Cannot have duplicate values Cannot contain Null values Foreign Key: Column(s) in which the values must match those of a candidate key in another table. Null Values A Null value means that no data was entered This is different from a zero, space character or tab character A Null is often ambiguous. It could mean The column value is not appropriate for the specific row The column value is not decided The column value is unknown Each may have entirely different implications 3

4 Primary Key Dependency Functional Dependence: The attribute B is functionally dependent on the attribute A if each value in column A determines one and only one value of B. Full Functional Dependence: When the primary key is composite, then the other columns must be identified by the entire key and not just some of the columns that make up the key. Transitive Dependencies: Occurs when a non-key attribute can be used to identify other non-key attributes Determinants The attribute (or attributes) that we use as the starting point (the variable on the left side of the equation) is called a determinant ValueA determines ValueB (ValueA) ValueB Determinant Dependency Example author bookname genre Walt Whitman Leaves of Grass Poetry walt@books.com Leo Tolstoy Anna Karenina Literary Fiction toldstoy@books.com Leo Tolstoy A Confession Religious Autobiography toldstoy@books.com Full Functional Dependence? 4

5 Dependency Example bookid author bookname genre Walt Whitman Leaves of Grass Poetry 7593 Leo Tolstoy Anna Karenina Literary Fiction 1827 Leo Tolstoy A Confession Religious Autobiography toldstoy@books.com Transitive Dependencies? Surrogate Key Sometimes there is not an existing key that fits A surrogate key is a unique, numeric value that is added to a relation to serve as the primary key Auto-Number is commonly used Surrogate key values have no meaning to users and are usually hidden on forms, queries and reports A surrogate key is often used in place of a composite primary key Integrity Adherence to moral and ethical principles; soundness of moral character; honesty. The state of being whole, entire, or undiminished: to preserve the integrity of the empire A sound, unimpaired, or perfect condition 5

6 Database Integrity Entity Integrity: Each table must have a column or combination of columns with unique values. This ensures that entities are uniquely identified in the database Referential Integrity: Column values in one table must match column values in a related table. This ensures valid links Database Integrity Constraints Entity Integrity Constraints No two rows of a table can have the same values of a primary key. Primary key values cannot be null Referential Integrity Constraints All foreign keys must either Must have a candidate key value in some row of a table with associated candidate key Or be null Key Usage Primary Keys are used to extract data from the database Example: Student ID is used to get student information Foreign Keys are used to create a link between tables when extracting data about different subjects Example: Student ID can be linked to classes taken 6

7 Relational Operators SELECT: Selects the desired fields from a table JOIN: Joins fields from two or more different tables UNION: Combines all rows from two tables Excludes duplicate rows Table Joins Inner Join: Returns all rows from both tables where there is a match Natural Join: Same as inner join but eliminates duplicate columns Non-equijoin: Joins two or more tables based on a specified column value not equaling a specified column value in another table Outer Join: Used to return all rows that exist in one table and only the matching records in the second table Self Join: Used to join a table to itself Indexing the Data Used to improve access and sorting performance Indexes can be constructed for a single column or groups of columns Useful for retrieving data and joining tables They have different effects for Selecting data Inserting data Updating data Deleting data 7

8 Conclusion Data models are used to facilitate interaction between designers, developers, and users Must understand the three levels of abstraction Proper Database design is critical!!! Next Time: Download PowerPoint slides about DBMS Design Be ready to discuss Database Life Cycle (DBLC) 8