Selecting the Appropriate Field Type for Data

The data type you select for each field can greatly affect the performance and functionality of an application. Several factors can influence your choice of data type for each field in a table:

• The type of data that's stored in the field

• Whether the field's contents need to be included in calculations

• Whether you need to sort the data in the field

• The way you want to sort the data in the field

• How important storage space is to you

The type of data you need to store in a field has the biggest influence on which data type you select. For example, if you need to store numbers that begin with leading zeros, you can't select a Number field because leading zeros entered into a Number field are ignored. This rule affects data such as zip codes (some of which begin with leading zeros) and department codes.

If the contents of a field need to be included in calculations, you must select a Number or Currency data type. You can't perform calculations on the contents of fields defined with the other data types. The only exception to this rule is Date data type fields, which you can include in date/time calculations.

You must also consider whether you will sort or index the data in a field. You can't sort the data in OLE Object and Hyperlink fields, so you shouldn't select these field types if you must sort or index the data in the field. Furthermore, you must think about the way you want to sort the data. For example, in a Text field, a set of numbers would be sorted in the order of the numbers' leftmost character, then the second character from the left, and so on (that is, 1, 10, 100, 2, 20, 200) because data in the Text field is sorted as characters rather than numbers. On the other hand, in a Number or Currency field, the numbers would be sorted in ascending value order (that is, 1, 2, 10, 20, 100, 200). You might think you would never want data sorted in a character sequence, but sometimes it makes sense to sort certain information, such as department codes, in this fashion. Access 2003 gives you the ability to sort or group based on a Memo field, but it performs the sorting or grouping only based on the first 255 characters. Finally, you should consider how important disk space is to you. Each field type takes up a different amount of storage space on a hard disk, and this could be a factor when you're selecting a data type for a field.

Nine field types are available in Access: Text, Memo, Number, Date/Time, Currency, AutoNumber (known as Counter in Access 2.0), Yes/No, OLE Object, and Hyperlink. Table 8.1 briefly describes the appropriate uses for each field type and the amount of storage space each type needs.

Table 8.1. Appropriate Uses and Storage Space for Access Field Types

Field Type	Appropriate Uses	Storage Space
Text	Data containing text, a combination of text and numbers, or numbers that don't need to be included in calculations. Examples are names, addresses, department codes, and phone numbers.	Based on what's actually stored in the field; ranges from 0 to 255 bytes
Memo	Long text and numeric strings. Examples are notes and descriptions.	Ranges from 0 to 65,536 bytes.

Number	Data that's included in calculations (excluding money). Examples are ages, codes (such as employee IDs), and payment methods.	1, 2, 4, or 8 bytes, depending on the field size selected (or 16 bytes for Replication ID).
Date/Time	Dates and times. Examples are date ordered and birth date.	8 bytes.
Currency	Currency values. Examples are amount due and price.	8 bytes.
AutoNumber	Unique sequential or random numbers. Examples are invoice numbers and project numbers.	4 bytes (16 bytes for replication ID).
Yes/No	Fields that contain one of two values (for example, yes/no, true/false). Sample uses are indicating bills paid and tenure status.	1 bit.
OLE Object	Objects such as Word documents or Excel spreadsheets. Examples are employee reviews and budgets.	0 bytes to 1GB, depending on what's stored within the field.
Hyperlink	Text or a combination of text and numbers, stored as text and used as a hyperlink for a Web address (uniform resource locator [URL]) or a universal naming convention (UNC) path. Examples are Webpages and network files.	0 to 2,048 bytes for each of the three parts that compose the address (up to 64,000 characters total).

The most difficult part of selecting a field type is knowing which type is best in each situation. The following detailed descriptions of each field type and when you should use them should help you with this process.

Text Fields: The Most Common Field Type

Most fields are Text fields. Many developers don't realize that it's best to use Text fields for any numbers that are not used in calculations. Examples of such numbers are phone numbers, part numbers, and zip codes. Although the default size for a Text field is 50 characters, you can store up to 255 characters in a Text field. Because Access allocates disk space dynamically, a large field size doesn't use hard disk space, but you can improve performance if you allocate the smallest field size possible. You can control the maximum number of characters allowed in a Text field by using the FieldSize property.

Memo Fields: For Long Notes and Comments

A Memo field can store up to 65,536 characters of text, meaning that it can hold up to 16 pages of text for each record. Memo fields are excellent for any type of notes you want to store with table data. Remember that in Access 2003 you can sort by a Memo field.

Number Fields: For When You Need to Calculate

You use Number fields to store data that you must include in calculations. If currency amounts are included in calculations or if calculations require the highest degree of accuracy, you should use a Currency field rather than a Number field.

The Number field is actually several types of fields in one because Access 2003 offers seven sizes of numeric fields. Byte can store integers from 0 to 255, Integer can hold whole numbers from –32768 to 32767, and Long Integer can hold whole numbers ranging from less than –two billion to just over two billion. Although all three of these sizes offer excellent performance, each type requires an increasingly large amount of storage space. Two of the other numeric field sizes, Single and Double, offer floating decimal points and, therefore, much slower performance than Integer and Long Integer. Single can hold fractional numbers to 7 significant digits; Double extends the precision to 14 significant digits. Decimal, a numeric data type that was introduced with Access 2002, allows storage of very large numbers and provides decimal precision up to 28 digits! The final size, Replication ID, supplies a unique identifier required by the data synchronization process.

Date/Time Fields: For Tracking When Things Happened

You use the Date/Time field type to store valid dates and times. Date/Time fields allow you to perform date calculations and make sure dates and times are always sorted properly.

Access actually stores the date or time internally as an 8-byte floating-point number. Access represents time as a fraction of a day.

Currency Fields: For Storing Money

The Currency field is a number field that is used when currency values are being stored in a table. A Currency field prevents the computer from rounding off data during calculations. It holds 15 digits of whole dollars, plus accuracy to one-hundredth of a cent. Although very accurate, this type of field is quite slow to process.

AutoNumber Fields: For Unique Record Identifiers

The AutoNumber field in Access 2003 is equivalent to the Counter field in Access 2.0. Access automatically generates AutoNumber field values when the user adds a record. In Access 2.0, counter values have to be sequential. The AutoNumber field type in Access 2003 can be either sequential or random. The random assignment is useful when several users are adding records offline because it's unlikely that Access will assign the same random value to two records. A special type of AutoNumber field is a Replication ID. This randomly produced, unique number helps with the replication process by generating unique identifiers used to synchronize database replicas.

You should note a few important points about sequential AutoNumber fields. If a user deletes a record from a table, its unique number is lost forever. Likewise, if a user is adding a record but cancels the action, the unique counter value for that record is lost forever. If this behavior is unacceptable, you can generate your own counter values.

Yes/No Fields: For When One of Two Answers Is Correct

You should use Yes/No fields to store a logical true or false. What Access actually stores in the field is -1 for yes, 0 for no, or Null for no specific choice. The display format for the field determines what the user actually sees (normally Yes/No, True/False, On/Off, or a third option—Null—if you set the TripleState property of the associated control on a form to True). Yes/No fields work efficiently for any data that can have only a true or false value. Not only do they limit the user to valid choices, but they also take up only 1 bit of storage space.

OLE Object Fields: For Storing Just About Anything

OLE Object fields are designed to hold data from any OLE server application that is registered in Windows, including spreadsheets, word processing documents, sound, and video. There are many business uses for OLE Object fields, such as storing resumes, employee reviews, budgets, or videos. However, in many cases, it is more efficient to use a Hyperlink field to store a link to the document rather than store the document itself in an OLE Object field.

Hyperlink Fields: For Linking to the Internet

Hyperlink fields are used to store uniform resource locator addresses, which are links to Web pages on the Internet or on an intranet, or UNC paths, which are links to a file location path. The Hyperlink field type is broken into three parts:

• What the user sees

• The URL or UNC

• A subaddress, such as a range name or bookmark

After the user places an entry in a Hyperlink field, the entry serves as a direct link to the file or page it refers to.