RDBMS Definition

Relational databases store data in tables (relations) that are two dimensional. The tables have rows (records or objects) and columns (fields or attributes). Data items at an intersection of a row and a column are called a cell and consist of attribute values. Data stored is simple data such as integers, real numbers or string values. Multiple values may not be stored in one cell. Relational database tables are "normalized" so data is not repeated more often than necessary. All table columns depend on a primary key (a unique value in the column) to identify the column. Once the specific column is identified, data from one or more rows associated with that column may be obtained or changed.

Relational databases are sets of tables. One table file is not a relational database. A relational database server is not the same as a relational database. A relational database can be a file with sets of tables. The relational database server includes the ability to service requestesto get or change data from remote clients.

Relational database servers use Structured Query Language (SQL), as a data manipulation language to interface between itself and the clients. SQL is the standard for getting and storing data in an RDBMS. For information about SQL, see the "Beginner's SQL Guide".

Relational database servers provide:

Data Management
Transaction processing
Data integrity - Provides for multiple access at the same time (concurrency) between multiple processes/users. This is done so data is not displayed nor saved in a fashion where one change is lost. Various locking mechanisms are used to support this.
Data backup and recovery.
Data security - Provides for user authentication, and levels of data access privileges.

Primary Keys

In relational databases, the data is not arranged in any particular order in tables. The data in tables requires keys for identification of rows. Each table has rows and columns. Sets of values in a row may describe a particular item such as customers. Consider the following table:

Customers

Customer ID	Name	Street Address	City	Zip Code	Phone	e-mail
112304	John Brown	123 Straight Lane	Paradise	MI	49555	johnb@heaven.net
134056	Tim Smith	321 Curved Ave	Hollywood	MD	10255	tims@eastnet.net
234902	Sue Jones	213 Hill Road	Knoxville	TN	23555	suej@nicenet.net
092387	George Abernathy	555 North Road	Smalltown	FL	67555	georgea@mynet.net
187462	Tom Jenkins	666 South Lane	Cold	MN	55555	tomj@networks.com
108976	Kim Adams	456 Peachtree Lane	Atlanta	GA	54555	kima@bestnet.net
059385	Chris Christopher	859 East Road	Nicetown	MS	45555	chrisc@bignet.com
140763	Jim Thompson	307 West Ave	Bigcity	ND	65555	jimt@fastnet.net

Columns include Customer ID, Name, Street Address, City and so forth. Each column has a different set of information about all customers such as their phone number. This information could be considered customer characteristics or attributes. Each row has all information about one particular customer. Each row must have a unique means of identification. We could have used the customer name to identify the customer, but it is possible that more than one customer may have the same name. Therefore we are using unique customer identifiers. The Customer ID is the key used to identify individual customers. The column to be used as the key must be identified to the relational database. This is called the primary key.

Foreign Keys

Let's say we're running a store with the customers listed above. The first table below lists items for sale. The second table lists orders that have been placed in the store, and the third table lists items ordered for an individual order. If we want to print out what items customer 234902 ordered we do the following:

Go to the orders table and get all orders customer 234902 ordered.
For each order found (1 order #190389575). The order number is considered to be a foreign key to another table and column which is to table "Individual Order 190389575" and Item ID.
Go to the "Individual Order 190389575" table and get each item ID that was ordered. Each item ID is a foreign key to the "Store Items" table and Item ID column.
Go to the store items table, find each item ID and print out each item attribute such as price, description, and quantity purchased (from the individual order table).

Store Items

Item ID	Description	Price
000234567	Small Flower Pot	2.36
000018901	Potting Soil	5.97
001034654	Geranium Seed	1.50

Orders

Order ID	Customer ID	Subtotal	Tax	Total
190389575	234902	16.16	0.82	16.98
109748230	187462	2.37	0.12	2.49
208949023	059385	104.23	5.22	109.45
103792034	187462	40.00	2.00	42.00
123048938	134056	10.00	0.50	10.50

Individual Order 190389575

Item ID	Quantity
000234567	2
000018901	2
001034654	1

Therefore foreign keys are used to access data in related tables.

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