24  What is a Database?

A Database is an organized collection of data that is designed to be accessed and manipulated by computer programs.

A Database Management System (DBMS) is the software that is used to add, remove and manage the data in a database. While a database is the term we use to refer to the actual data.

You can think of a database and a DBMS similar to the way you think about the Excel software program VS an Excel file. The DBMS is similar to the Excel software program, while a database is similar to an Excel file.

A single “Database Management System” (i.e. the software) is used to control many different “databases”.


• Individual databases contain information about a particular topic.

• EXAMPLE: A telephone company may have the following different databases (as well as many others). These different databases can all be controlled through one DBMS program:

– Human Resources Database: contains information about employee name, titles, hire dates, salaries, pension and benefit information, 401K data, etc

– Customer and billing database: information about customer names and addresses, billing information, calling patterns, etc

– Assets database: information about real estate that the company owns and leases, cell phone towers and leased lines, etc.

– Marketing database: information about marketing campaigns, advertising rates, locations of billboards that are being used, times/stations and rates for radio and TV ad spots, data from focus groups, etc.

– etc.

People who use databases

• Technical IT Professionals (e.g. programmers, database administrators)

– IT Professional such as programmers and database administrators need to understand the in-depth issues about databases. These professionals create and maintain the program that store their information in databases.

• End Users

– Most business people do not usually use a database directly. Rather, business people use programs geared to their specific business needs. “Behind the scenes” those programs store or retrieve data from a database in response to the user’s actions.

– It is sometimes very helpful for business professionals to be able to access information in databases directly as an IT professional would. This requires some basic knowledge of the SQL language.

• Business Analysts (BA)

– When a new program needs to be created, the business analysts work with both the “end users” and the “technical IT professionals” to design the programs. The BAs don’t actually create the computer program code. Rather, the BAs work on the design of what the software should do. BAs must also understand database concepts.

• (See architecture diagram later in this presentation …)

Features of Databases

Database vs. Excel

• Medium to large sized companies typically have many different databases that are used to track the many types of data that their business depends on.

• It is arguably more complicated to implement a database solution than an Excel spreadsheet.

• Many students wonder what advantages a database has over an Excel file for storing data.

• The following slides list some features of modern databases that cannot be easily accomplished with a program such as Excel

• The combination of these reasons makes Excel a poor choice for many types of applications. The complexity and expense of implementing a database solution is more than justified for many business scenarios.

Features of Modern Databases

Access to the data by computer programs (not just people)

Databases are designed for automated storage, retrieval and manipulation of the data. Computer programs can be designed to access the data, not just people. Websites, apps and desktop programs can be built that insert, retrieve and update information from the database.

• Excel was not designed to be used in this way.

Simultaneous access to the data by multiple users.

• Databases are designed so that the data can be accessed over the Internet or a local computer network by several users/programs at the same time.

• Excel was not designed for this. Newer versions of Excel have limited capabilities for “sharing” workbooks but this feature is limited to a few users and doesn’t scale to Internet style access.

Able to process HUGE amounts of data.

• Generally, the amount of data that can be stored in a database is limited only by the amount of disk space on the computer.

• Excel is limited to about 1million rows per sheet and 16 thousand columns. This may sound like a lot of data but (a) Excel can get very slow when that much data is in a spreadsheet and (b) that is NOT enough data for truly large scale applications, such as storing data for Amazon.com or the New York Stock Exchange.

Efficient (i.e. fast) storage, retrieval and manipulation of the data.

• Databases can be designed to store and retrieve vast amounts of information extremely quickly.

• Excel works well when an individual user manipulates the data in a spreadsheet one cell at a time. However, Excel does not scale to hundreds or thousands of changes a minute.

Flexible querying capability

• Using a language such as SQL the information in a database can quickly be summarized, combined and reorganized in an unlimited number of ways.

• Excel has limited capabilities for summarizing and reorganizing data (e.g. subtotals, pivot tables, etc). However, the flexibility offered by databases far surpass the features that are offered by Excel.

Features of Modern Databases (continued)

Durable data.

• Databases have features to ensure that data will remain valid and uncorrupted even in the face of catastrophic errors such as a power outage or a hard-drive-crash. For example, in the wake of a power outage, data that is stored in a bank’s databases are guaranteed to be retrievable.

• Excel does not have guarantees which prevent corruption of the data in the wake of power outages, hard drive crashes or similar catastrophic events.

Validation features.

• Databases have features for preventing “bad data” from being entered into the database (e.g. typos or illogical information such as a person’s age being recorded as 237, or as a word such as “blue”, etc). Databases can be setup with rules to describe what is considered valid data. Invalid data will not be accepted into the database.

• Excel has some limited ability to control what can be entered into specific cells. However, many would argue that these features are difficult to use efficiently. Most Excel users don’t even know that these features exist and don’t use them.


• Databases have abilities to allow some users to access specific data but not other users.

Authentication” means that before accessing the data, a user must “login” or otherwise identify who they are. If a user has already logged into the operating system (e.g. windows) then many databases can identify the user based on the information he used to log into the operating system.

Authentication is necessary to allow the DBMS to enforce security of the data. The database can be setup with rules to indicate which users can access which data. For example a database might store information about employees in a company. The names and telephone numbers might be available to every employee in the company. However, salary data might only be available to employees in the human resources department.

• Excel has very limited capability to restrict a user from accessing specific cells, unless that user has the password for the spreadsheet. However, Excel’s security feature is dwarfed by the vast number of security features of a modern database system.

Relational Databases
vs Other types of Databases

Relational Databases vs other types of databases

• Database management systems (and databases) are categorized by the formats in which they organize information.

• A “Relational database management system” (RDBMS) is software that is used to manage a “relational database”. These types of databases organize their information in rows and columns similar to how information is organized in an Excel document.

– Relational databases are sometimes known as SQL databases since the creation and manipulation of data in a relational database is done using the SQL computer language. (more on this later …)

– Relational databases are very popular and have been around since the 1970s.

• Other categories of databases include different types of “NoSQL” databases. NoSQL databases generally do NOT store their information in rows and columns, but in other structures. Some types of NoSQL databases include

– Document databases (e.g. “MongoDB”)

– Graph databases (used to store “graphs” such as Facebook’s “friends graph”)

– Key-Value databases (stores values that can be looked up with a “key value” – e.g. key=SSN, value=person’s name)

• The remainder of this presentation will focus primarily on “Relational Databases”.

Relational DBMS Products

Current Relational DBMS Products

• Some of the currently used RDBMS products:

MySql: “open source” DBMS. Can be downloaded and used for free. Powerful. Companies can purchase a technical support license from MySql, inc. Many many of the websites on the Internet use MySql to store information.

PostgreSQL – another “open source” DBMS, similar to MySql

Microsoft SQL Server – Microsoft’s “high-end” DBMS. More powerful than Access (i.e. faster, can handle more data, more users, better security features, etc).

Microsoft Access – Microsoft’s “low-end” DBMS. Used for smaller applications. Cheaper than SQL Server to buy/license. Includes built in features for building apps. Not really used for large applications.

Oracle – first commercially successful Relational DBMS. Very powerful. Popular for large DB applications in fortune 500 companies.

IBM Informix

– many others …

Database Client Server Architecture

Database Diagram

• See the diagram on the next slide. It will be discussed throughout this section.

• Don’t worry if you don’t understand the diagram at first. The rest of the slides in this section will explain the details of the diagram.

• see diagram on next slide …

Architecture Diagram

Database Architecture Diagram

• The diagram on the previous slide shows the organization of a typical database system.

• Two computers in the diagram:

– The two computers on the diagram communicate with each other.

– The computer on the left is running an “application program”.

– The application program in this picture is used to look up telephone numbers for specific people.

– The telephone numbers are actually stored on the other computer. The telephone number lookup program will communicate with the computer on the right to get the telephone numbers.

– Programs that get information from other programs are known as “client programs.

– The computer on the right is running a DataBase Management Server program (DBMS).

– The DBMS maintains the database that stores the telephone numbers.

– This computer is shown without a screen because the human user doesn’t use this computer directly.

– Rather this computer is accessed indirectly by the telephone number lookup program.

– A computer that runs software that is used by other programs is known as a “server computer”. The program that it runs is known as a “server program”.

• See next slide …

About the Diagram: The database

• The actual database is shown on the diagram inside the cylinder shape, below the server computer (i.e. the computer on the right).

• The cylinder that surrounds the data on the diagram represents the server computer’s hard drive. Even though the hard drive is usually “inside” the server computer, it is often shown on such diagrams separately.

Complete Explanation of Architecture Diagram

• Complete Explanation of Architecture Diagram

– Step 1: The user runs an application program that allows him to look up people’s telephone numbers. The user types in a name to lookup and then clicks the “Lookup” button.

– Step 2: The application needs to get the information so the application program sends a request to the DBMS program over the computer network to lookup the information in the database. (This request is actually expressed using the SQL programming language. SQL is discussed later in this presentation)

– Step 3: The DBMS looks up the telephone number in the database.

– Step 4: The DBMS sends the information back to the client application program.

– Step 5: The application program displays the answer to the user (not shown in the diagram). It appears to the user that the application program “knows” the telephone number. However, really, the application program just asked for this information from the database management system.

Client and Server programs can run on the same computer or on different computers

• Note that it is possible to have the application program (AKA “client program”) and the DBMS program (AKA “server program”) on the same computer.

• However for large database applications used by businesses, the DBMS is usually on a different computer than the application program is running on.

Other Types of Database Clients

• The following are different type of systems that interact with databases

mobile apps that access data – most iOS or Android apps that accesses data are actually communicating over the Internet to a server program that uses a database to store and retrieve data.

websites that access a lot of data – e.g. facebook, amazon, google – all of these websites use databases to store and retrieve the information that you see. For example, your list of facebook friends and the data on your facebook wall are stored in databases, all product information on amazon is stored in a database, the lists of websites on the internet and what information they store is stored in databases maintained by google. When you browse those websites the “web server” program that you communicate with in turn uses a “database server” program to store and retrieve the information that you see.

ATM machine– data is stored on the bank’s database. The data is accessible from any ATM machine.

Point of Sale – when you buy items in a supermarket and the cashier “scans” the item – the scanner and the checkout system acts as a client to the database server. The database sends product and pricing information back to the checkout system to produce the bill. The database server also updates its inventory records to reflect the fact that the items have been purchased.

”EZ-Pass” – when a car passes through an “ez-pass” toll booth, the information about the car is recorded in a database which is then used to generate billing information.

– etc.

Structured Query Language (SQL)

• The application program uses a special computer language, called Structured Query Language (SQL), to communicate with the DBMS program.

• SQL is often pronounced as “sequel”

• Users of the application program do NOT need to understand SQL. It is the computer programmers who create application programs who must understand how to write SQL.

SQL : Inserts, Updates, Deletes, Queries, …

• The most common types of SQL requests are:

Inserts: Requests from the application program to add information to the database (DB).

Deletes: Requests from the application program to remove information from the DB.

Updates: Requests from the application program to modify some information that already exists in the DB.

Queries: (these are also known as “selects”) Requests from the application program to retrieve specific information from the DB.

The diagram above shows the application asking the DMBS for Joe Smiths telephone number. This is a simple query that returns a single piece of information. Queries can be much more complex and return a lot of information.

An example of a more complex query might be – find the list of names and telephone numbers for those people whose last name begins with “S” and whose telephone number is in the 917 area code.

Many Clients at the Same Time

• The diagram above only shows a single client computer. In reality it is very common for many different client computers to run application programs at the same time.

• For example, it is likely that there are many different people each running a separate copy of the telephone number lookup program on their own computer. In this case there would be many different telephone number client computers all communicating with the one DataBase Management System server program.

Different types of client application programs.

• The diagram above only showed one type of client application program, i.e. the telephone number lookup program. However, it is common for information contained in a single database to be used by different types of application programs simultaneously.

• Example: A database that tracks information about planes and flights for an airline might be accessed by all of the following different types of application programs simultaneously:

– A program used by ticketing agents to sell tickets for the flights

– A different program used by the airplane maintenance staff to determine which planes need to be maintained based on how many miles they have flown so far

– A third program used by the payroll department to calculate paychecks based on the number of hours and miles flown by the crew.

– Another program may run on mobile phones to allow customers to look up flight delay information.

– etc.