Components of a Database System

Four basic components.

Users

The reason that the database exists.
Enter data, perform queries, ...

Application

A high level program, probably produced in house
Contains customized business knowledge

DBMS

Usually a professional software package.
Specialized knowledge on how to organize, store and search data

Database

The collection of data, tables, relations ...

But SQL is a fifth element which shows up at various stages.

The Application Program

Basic Functions
- Create and process forms.
  - The forms used by the business
  - Order form, restock form, ....
  - All the paperwork
  - In some cases, the application becomes the forms.
  - These are unique to each business
  - These have nothing to do with how the data is organized or stored.
- Process User Queries
  - Ask the DBMS for data
  - This is probably done in SQL, but most users don't know SQL
  - So the application is required to translate user input into SQL
  - And present the results back to the user.
- Create and process reports
  - Companies live on reports of the status of the company
  - What should be produced today?
  - What should be ordered?
  - How many units were sold last week?
  - These are all examples of regular reports that need to be generated.
  - They are all customized to the individual business
  - They all require data from the database
- Execute application logic
  - When does the company need to reorder parts?
  - When should a production line be shutdown?
  - Should a student be permitted to sign up for 21 credits?
  - These are all examples of application logic
  - When data is entered into the database, or an attempt is made to enter data, some other action may be required.
  - This again, is all local business logic
- Control the application.
  - This is related to the previous set.
  - Who has access to what.
  - For example, not everyone should be able to read salaries
  - Control multiple dbms operations
  - For example, require that either an entire order be entered or nothing.

DBMS

Create database
Create tables in a database
Create supporting structures (index, functions, views)
- These three are all base database creation functions.
Modify database data (insert, delete, update)
Read database data
Maintain database structures
- These are all associated with storing and retrieving data from the database.
Enforce rules
- referential integrity constraints.
- Prevent good data from becoming corrupt.
Control concurrency
- An OS related task, make sure that some sense of data manipulation order is maintained.
- Readers/Writers problem
- Plus other OS like things
- Security
- Disk usage
- Caching
Backup and Recovery

Database

Databases have integrated tables.
- The table contains the data
- But also the relations between the data.
Databases are self-describing
- They contain a description of how the table is built
- And a description of the data in the table.
- Metadata - data about data.
- This column is an integer.
- There is also information on users, permissions, ...