Introduction to DBMS
DATABASE MANAGEMENT SYSTEMS:
- It is the collection of interrelated data.
- A set of programs are used to access the data.
- DBMS contains information about a particular enterprise
- DBMS provides an environment that is both convenient and efficient to use.
★ Banking: all transactions
★ Airlines: reservations, schedules
★ Universities: registration, grades
★ Sales: customers, products, purchases
★ Manufacturing: production, inventory, orders, supply chain
★ Human resources: employee records, salaries, tax deductions
Purpose of Database System
In the early days, database applications were built on top of file systems
Drawbacks of using file systems to store data:
★ Data redundancy and inconsistency
✔ Multiple file formats, duplication of information in different files
★ Difficulty in accessing data
✔ Need to write a new program to carry out each new task
★ Data isolation — multiple files and formats
★ Integrity problems
✔ Integrity constraints (e.g. account balance > 0) become part of program code
✔ Hard to add new constraints or change existing ones
★ Atomicity of updates
✔ Failures may leave database in an inconsistent state with partial updates carried out
✔ E.g. transfer of funds from one account to another should either complete or not happen at all
★ Concurrent access by multiple users
✔ Concurrent accessed needed for performance
✔ Uncontrolled concurrent accesses can lead to inconsistencies
– E.g. two people reading a balance and updating it at the same time
★ Security problems
■ 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
name : string;
street : string;
city : integer;
end;
■ View level: application programs hide details of data types.
Views can also hide information (e.g., salary) for security
purposes.
Instances and Schemas Instances and Schemas
■ Similar to types and variables in programming languages
Schema – the logical structure of the database
★ e.g., the database consists of information about a set of customers and
accounts and the relationship between them)
★ Analogous to type information of a variable in a program
★ Physical schema: database design at the physical level
★ Logical schema: database design at the logical level
Instance – the actual content of the database at a particular point in time
★ Analogous to the value of a variable
Physical Data Independence – the ability to modify the physical schema
without changing the logical schema
★ Applications depend on the logical schema
★ 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.
Data Models
■ A collection of tools for describing
★ data
★ data relationships
★ data semantics
★ data constraints
■ Entity-Relationship model
■ Relational model
■ Other models:
★ object-oriented model
★ semi-structured data models
★ Older models: network model and hierarchical model
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