Attachment data types are available only in. You can attach many kinds of files to a record. However, some file types that may pose security risks are blocked. As a rule, you can attach any file that was created in one of the Microsoft Office programs. You can also attach log files. For a list of supported image file formats, see the table later in this section. The label text that is displayed for this field by default in forms, reports, and queries.
If this property is empty, the name of the field is used. Any text string is allowed. Access supports the following graphic file formats without the need for additional software being installed on your computer. In addition, file names must follow the following guidelines:. Names cannot contain the following characters: question marks? Purpose Use an AutoNumber field to provide a unique value that serves no other purpose than to make each record unique. The most common use for an AutoNumber field is as a primary key, especially when no suitable natural key a key that is based on a data field is available.
An AutoNumber field value requires 4 or 16 bytes, depending on the value of its Field Size property. Suppose that you have a table that stores contacts' information. You can use contact names as the primary key for that table, but how do you handle two contacts with exactly the same name? Names are unsuitable natural keys, because they are often not unique. If you use an AutoNumber field, each record is guaranteed to have a unique identifier.
Note: You should not use an AutoNumber field to keep a count of the records in a table. AutoNumber values are not reused, so deleted records can result in gaps in your count. Moreover, an accurate count of records can be easily obtained by using a Totals row in a datasheet.
Determines the amount of space that is allocated for each value. For AutoNumber fields, only two values are allowed:. This is the default value. You should not change this value unless you are creating a replication ID field. Note: Replication is not supported in databases that use a new file format, such as. This setting makes AutoNumber fields compatible with other Long Integer Number fields when they are used in relationships or joins.
Each field value requires 4 bytes of storage. Do not use this value unless you are working in or implementing the design of a replicated database. Determines whether AutoNumber field increments with each new value or uses random numbers. Select one of the following:. Increment Starts with the value 1 and incrementally increases by 1 for each new record. Random Starts with a random value and assigns a random value to each new record. Values are of the Long Integer field size, and range from -2,,, to 2,,, If you are using an AutoNumber field as a primary key or as a Replication ID, you should not set this property.
Otherwise, choose a number format that meets your specific needs. Yes No duplicates Creates a unique index on the field. Yes Duplicates OK Creates a non-unique index on the field. No Removes any index on the field. Note: Do not change this property for a field that is used in a primary key. Without a unique index, it is possible to enter duplicate values, which can break any relationships in which the key is a part. Although you can create an index on a single field by setting the Indexed field property, some kinds of indexes cannot be created in this manner.
For example, you cannot create a multi-field index by setting this property. Purpose Use to store the results of a calculation. Note, Calculated data types were first introduced in Access Calculated data types are available only in. The result of this calculation will be stored in the calculated column. If this column has been saved, then only saved columns can be used in this expression. Determines the way that the field appears when it is displayed or printed in datasheets or in forms or reports that are bound to the field.
You can use any valid number format. In most cases, you should set the Format value to match the result type. Data in a Currency field is not rounded off during calculations. A Currency field is accurate to 15 digits to the left of the decimal point and 4 digits to the right. Each Currency field value requires 8 bytes of storage. In most cases, you should set the Format value to Currency. Displays editing characters to guide data entry. Supplies an expression that must be true whenever you add or change the value in this field.
Use in conjunction with the Validation Text property. Enter a message to display when a value that is entered violates the expression in the Validation Rule property. Determines the way that the field appears when it is displayed or printed in datasheets, or in forms or reports that are bound to the field. You can use a predefined format or build your own custom format. General Date By default, if the value is a date only, no time is displayed; if the value is a time only, no date is displayed.
This setting is a combination of the Short Date and Long Time settings. Example: Saturday, April 3, Medium Date Displays the date as dd-mmm-yyyy. Example: 3-Apr Long Time Same as the setting on the Time tab in the regional settings of Windows. Example: PM. Short Time Displays the time as hours and minutes separated by the time separator, by using a hour clock.
Example: Lists of components that you can use in custom formats. Type any combination of the following components to build a custom format. For more information on Windows regional settings, see Windows Help. Note: Separators are set in the regional settings of Windows. For example, hh:mm. Any short string of characters, enclosed in quotation marks "" Custom separator. Quotation marks are not displayed. For example, "," displays a comma. For example, PM. For example, pm. For example, P.
For example, p. Enter a fractional precision to specify the number of digits to the right of the decimal point Allows entry by setting to Yes of a zero-length string "" in a Hyperlink, Text, or Memo field. Yes Tracks changes.
To view the field value history, right-click the field, and then click Show column history. Warning: Warning Setting this property to No deletes any existing field value history. You can define a custom format for a Hyperlink field. Purpose Use to store a block of text that is more than characters long and is formatted text.
Note, beginning in Access Memo data type has been renamed to Long Text. Warning: Setting this property to No deletes any existing field value history. You can define a custom format for a Memo field. The field size defines the maximum number of characters a field can contain. For example, the Instructor ID field contains 6 characters and thus has a field size of 6. The type of data in a field is an important consideration.
The image to your right identifies the data types for fields in the Instructor and Student files. The data type specifies the kind of data a field can contain and how the field is used. Common data types include:. A record is a group of related fields.
For example, the Student ID field uniquely identifies each student because no two students can have the same Student ID. In some tables, the primary key consists of multiple fields, called a composite key. For example, the primary key for the Schedule of Classes file could consist of the fields Semester Code, Class Code, and Class Section, which together would uniquely identify each class listed in a schedule.
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