Once you have created entities representing the business objects you want to represent in your database, you need to build tables to store the data. Tables are made up of rows and columns. Each column corresponds to an attribute of the entity to which the table refers. Every row represents an instance of the entity described by the table. Specific data relating to each attribute is stored in the rows corresponding to each column.

The following interactive graphic shows a table that contains information about personal computers and points out the table's components.

For a relational database to function properly, table rows and columns must follow a number of rules grounded in the mathematical discipline of set theory . The next few lessons review those rules (in non-mathematical language whenever possible), but for now it is enough to know that the rules exist and that they are practical.
Because relational databases are based on set theory, you may encounter the formal terms for tables and their components in the literature of set theory. Here are the terms you're most likely to see:

1. Tables are referred to as relations .
   
2. Columns are referred to as attributes or fields .
   
3. Rows are referred to as tuples (pronounced "tup-pulls") or records.
   record: A particular instance of the subject of a table.

There are two basic types of tables in a relational database:

1. base table: A table stored in a database.
2. virtual table: A table stored in the computer’s memory. Virtual tables themselves are not stored in the database; rather, the definition of the view is stored and given a name. Users call up that name, and the view is created (from base tables) on the fly.
   When a user closes the view, it "disappears" from memory, only to be recreated the next time its name is invoked.

Base tables are permanent tables you create in the RDBMS ( Relational Database Management System), while virtual tables are temporary tables the RDBMS creates in response to user commands.
For example, if you enter a command to find the first name and last name of every customer who has placed an order in the past month, the RDBMS writes those names to a virtual table which it stores in system memory but does not write to disk.
Relational Database Management System (RDBMS): A software package that manages and provides access to a database. These packages follow Codd’s 12 rules of relational databases and normally use SQL to access data.

Before moving to the next lesson, click the Quiz link below to check your understanding of relational constructs and the characteristics of tables.
Relational Tabel Characteristics - Quiz