COP 4710 Study Guide - Fall 2018, Comprehensive Midterm Notes - Database Schema, Debtor, Unique Key
12 Oct 2018
School
Department
Course
Professor
COP 4710
MIDTERM EXAM
STUDY GUIDE
Fall 2018
Database management system(DBMS):
*DBMS contains info about particular enterprise
-collection of interrelated data
-set of prgrm to access the data
-an environment that is both convenient and efficient to use
*Database application:
-Banking;Airlines;Universities; Sales; Online retailers;
*DB touches all aspects of our lives
Purpose of Database Systems
*In the early days, database applications were built directly on top of file systems
*Drawbacks of using file systems to store data:
z Data redundancy and inconsistency
Multiple file formats, duplication of information in different files
z Difficulty in accessing data
Need to write a new program to carry out each new task
z Data isolation — multiple files and formats z Integrity problems
z Integrity constraints (e.g. account balance > 0) become “buried” in program code rather
than being stated explicitly
Hard to add new constraints or change existing ones
z Atomicity of updates
Failures may leave database in inconsistent state with partial updates carried out
Example: Transfer of funds from one account to another should either complete
or not happen at all
z Concurrent access by multiple users
Concurrent accessed needed for performance
Uncontrolled concurrent accesses can lead to inconsistencies
– Example: Two people reading balance and updating it at the same time
z Security problems
Hard to provide user access to some, but not all, data
→ *Database systems offer solutions to all the above problems
Levels of Abstraction Levels of Abstraction
*Physical level: describes how a record (e.g., customer) is stored.
*Logical level: describes data stored in database, and the relationships among the data.
type customer = record
customer_id : string;
customer_name : string;
customer _ street : string;
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customer_city : integer;
end
*View level: A way to hide: (a) details of data types and (b) information (such as an employee’s
salary) for security purposes.
View of data:
Instances and schemas:
*Similar to types and variables in programming languages
*Schema – the logical structure of the database → part that describes database
z Example: The database consists of information about a set of customers and accounts
and the relationship between them)
z Analogous to type information of a variable in a program
z Physical schema: database design at the physical level
z Logical schema: database design at the logical level
*Instance – the actual content of the database at a particular point in time → name of a
customer
z Analogous to the value of a variable
*Physical Data Independence – the ability to modify the physical schema without changing the
logical schema
z Applications depend on the logical schema
z In general, the interfaces between the various levels and components should be well
defined so that changes in some parts do not seriously influence others.
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Document Summary
An environment that is both convenient and efficient to use. *in the early days, database applications were built directly on top of file systems . *drawbacks of using file systems to store data: z data redundancy and inconsistency. Multiple file formats, duplication of information in different files z difficulty in accessing data. Hard to add new constraints or change existing ones z atomicity of updates. Failures may leave database in inconsistent state with partial updates carried out. Example: transfer of funds from one account to another should either complete or not happen at all z concurrent access by multiple users. Example: two people reading balance and updating it at the same time z security problems. Hard to provide user access to some, but not all, data . *database systems offer solutions to all the above problems. *physical level: describes how a record (e. g. , customer) is stored.
