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Microsoft Access database fields are created by entering a field name and a field data type in each row of the field entry area of the database table window. The field description is an option to identify the fields purpose; it appears in the status bar during data entry. After you enter each field's name and data type, you can specify how each field is used by entering properties in the property area. Before you enter any properties, however, you should enter all of you fields names and data types.
A field name identifies the field to both you and to Microsoft Access. For information regarding field names please see the article: Tables and Fields
After you name a field, you must decide what type of data the field will hold. Before you begin entering data, you should have a grasp of the data types that your system will use. Ten basic data types are shown in the table below; some data types (such as numbers) have several options:
|Data Type||Use For||Size|
|Text||Text or combinations of text and numbers, such as addresses. Also numbers that do not require calculations, such as phone numbers, part numbers, or postal codes.||Up to 255 characters. Microsoft Access only stores the characters entered in a field; it does not store space characters for unused positions in a Text field. To control the maximum number of characters that can be entered, set the FieldSize property.|
|Memo||Lengthy text and numbers, such as notes or descriptions.||Up to 64,000 characters.|
|Number||Numeric data to be used for mathematical calculations, except calculations involving money (use Currency type). Set the FieldSize property to define the specific Number type.||
1, 2, 4, or 8 bytes. 16 bytes for Replication ID (GUID) only.Click here for more information on changing Number field sizes.
|Date/Time||Dates and times.||8 bytes.|
|Currency||Currency values. Use the Currency data type to prevent rounding off during calculations. Accurate to 15 digits to the left of the decimal point and 4 digits to the right.||8 bytes.|
|AutoNumber||Unique sequential (incrementing by 1) or random numbers automatically inserted when a record is added.||4 bytes. 16 bytes for Replication ID (GUID) only.|
|Yes/No||Fields that will contain only one of two values, such as Yes/No, True/False, On/Off.||1 bit.|
|OLE Object||Objects (such as Microsoft Word documents, Microsoft Excel spreadsheets, pictures, sounds, or other binary data), created in other programs using the OLE protocol, that can be linked to or embedded in a Microsoft Access table. You must use a bound object frame in a form or report to display the OLE object.||Up to 1 gigabyte (limited by disk space).|
|Hyperlink||Field that will store hyperlinks. A hyperlink can be a UNC path or a URL.||Up to 64,000 characters.|
|Lookup Wizard||Creates a field that allows you to choose a value from another table or from a list of values using a combo box. Choosing this option in the data type list starts a wizard to define this for you.||The same size as the primary key field that is also the Lookup field; typically 4 bytes.|
For numeric data types, the field size enables you to further define the type of number, which in turn determines the storage size. The table below shows the seven possible settings in the Numeric Field Size property.
You should make the field size the smallest one possible; Access runs faster with smaller field sizes. Note the first three settings don’t use decimal points, but allow increasingly larger positive or negative numbers. Single and Double permit even larger numbers: Single gives you 7 decimal places, and Double allows 15. Use the Double setting when you need many decimal places or very large numbers.
|Setting||Description||Decimal Precision||Storage Size|
|Byte||Stores numbers from 0 to 255 (no fractions).||None||1 byte|
|Integer||Stores numbers from –32,768 to 32,767 (no fractions).||None||2 bytes|
|Long Integer||(Default) Stores numbers from –2,147,483,648 to 2,147,483,647 (no fractions).||None||4 bytes|
|Decimal||Stores numbers from -10^38 -1 through 10^38 -1 (.adp)
Stores numbers from-10^28 -1 through 10^28 -1 (.mdb)
|Single||Stores numbers from –3.402823E38 to
–1.401298E–45 for negative values and from
1.401298E–45 to 3.402823E38 for positive values.
|Double||Stores numbers from –1.79769313486231E308 to –4.94065645841247E–324 for negative values and from 1.79769313486231E308 to 4.94065645841247E–324 for positive values.||15||8 bytes|
|Replication ID||Globally unique identifier (GUID)||N/A||16 bytes|
Caution: If you convert a large FieldSize setting to a smaller one in a field that already contains data, you might lose data. For example, if you change the FieldSize setting for a Text data type field from 255 to 50, data beyond the new 50-character setting will be discarded.
If the data in a Number data type field doesn't fit in a new FieldSize setting, fractional numbers may be rounded or you might get a Null value. For example, if you change from a Single to an Integer field size, fractional values will be rounded to the nearest whole number and values greater than 32,767 or less than –32,768 will result in null fields.
Tip: You can use the Currency data type if you plan to perform many calculations on a field that contains data with one to four decimal places. Single and Double data type fields require floating-point calculation. Currency data type fields use a faster fixed-point calculation.