Appendix D. Using SQL Datatypes

As explained in Lesson 1, "Understanding SQL," datatypes are basically rules that define what data may be stored in a column and how that data is actually stored.

Datatypes are used for several reasons:

• Datatypes enable you to restrict the type of data that can be stored in a column. For example, a numeric datatype column will only accept numeric values.

• Datatypes allow for more efficient storage, internally. Numbers and date time values can be stored in a more condensed format than text strings.

• Datatypes allow for alternate sorting orders. If everything is treated as strings, 1 comes before 10, which comes before 2. (Strings are sorted in dictionary sequence, one character at a time starting from the left.) As numeric datatypes, the numbers would be sorted correctly.

When designing tables, pay careful attention to the datatypes being used. Using the wrong datatype can seriously impact your application. Changing the datatypes of existing populated columns is not a trivial task. (In addition, doing so can result in data loss.)

Although this lesson is by no means a complete tutorial on datatypes and how they are to be used, it explains the major datatype types, what they are used for, and compatibility issues that you should be aware of.