Chapter 1

Database Management System (DBMS)

• DBMS contains info about a particular enterprise (ex. an institution, a business)

  • Collection of interrelated data (ex. info on an employee’s address, benefit’s that employee’s have)

  • Set of programs to access the data

  • An environment that is both convenient and efficient to use

• Examples of Database Application include things like transactions in Banking or Reservations in Airlines - Databases touch on all aspects of our lives

• University Database Example

  • Application program examples

    • Add new students, instructors, and courses

    • Register students for courses, and generate class rosters

    • Assign grades to students, computer GPA and generate transcripts

  • In the early days, database applications were built directly on top of file systems

• Purpose of Database 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

      • Join - Merging two different files together to create one file

    • Integrity problems

      • 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

    • Atomicity of updates

      • Failures may leave database in an inconsistent state with partial updates carried out

      • Ex.: transfer of funds from one account to another should either complete or not happen at all

    • Concurrent access by multiple users

      • Concurrent access needed for performance

      • Ex.: Two people reading a balance and updating the balance by withdrawing money at the same time

    • 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

• Physical Level: describes how a record (e.g., customer) is stored.

• Logical Level: describes data stored in database, and the relationships among the data.

• View Level: applications programs hide details of data types. Views can also hide information (such as an employee’s salary) for security purposes.

Instances and Schemas

• Similar to types and variables in programming languages

• Schema - the logical structure of the databse

  • Ex.: The databases 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 a database at a particular 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

• Some examples of models include:

  • Relational Model

  • Entity-Relationship data model (mainly for database design)

  • Object-based data model (object oriented and object-relational)

  • Semistructured data model (XML)

• Other Older models include

  • Network model

  • Hierarchical model

Relational Model

• Details will be in Chapter 2, so why tf are we atlking about it now. idk picture in slides

• Type in the relational model is the domain where you pull values from. Ex.: The column name is the type of data that the column shows