Each row in a database table is made up of one or more columns. Each column contains a single piece of data, part of the complete record stored in the row. When a table is created, each of its columns needs to be defined. Defining columns involves specifying the column’s name, size, and data type. The data type specifies what data can be stored in a column.
Data types specify the characteristics of a column and instruct the database as to what kind of data can be entered into it. Some data types allow the entry of free-form alphanumeric data. Others restrict data entry to specific data, such as numbers, dates, or true or false flags. A list of common data types is shown in Table 5.1.