Columns |
Table columns are administered from the "Column" tab of each table's property page.
Each column must have a name that is unique within the table. Make sure that you choose a column name that is valid for the database that you are using.
To the right of the column name, there is a button with ellipses on
it. 
Use this button to map to another column that you've already defined in your project. The data type and length properties will then be shared among mapped columns, and changes in one table will be reflected everywhere the column is used. However, other properties (such as autonumber) are unique to the table to which the column belongs.
The datatype defines the type of data that will be stored in a column. Different databases have different datatypes. Depending on the data-type you select (and on your database platform), one or more of the column properties discussed below may not apply to the column you are designing. These properties will be un-enabled on the screen. For example, the Allow Zero-Length property only applies to data-types that contain text data and is only used by Microsoft Access databases.
If you convert your database to a different type, Visual Case will convert your columns to their equivalent datatype in the new database.
When a new record is added to a table, each column that has a default value will contain the default value unless a value is explicitely set by the database user. Columns that don't have defaults defined, will contain a value of null. This will result in an error if the database column doesn't allow null values.
Autonumber fields (often called identity fields) contain system-generated numbers which are always unique. You can only set up a column as an autonumber field if it has the correct datatype (Long Integer for Access databases or int for SQL databases). Autonumber fields cannot contain nulls.
You will only be able to view and set the length property if the data-type allows it. The length property may not apply to some data-types and may be fixed for others. The length for decimal data-types is defined in the Total and Fraction fields (see below).
Some numeric data-types allow you to specify the total number of digits that the number can contain and the number of fractional digits that a number can contain. Often databases use the terms scale and precision.
|
TOTAL |
FRACTION |
EXAMPLE |
|
4 |
1 |
123.4 |
|
4 |
0 |
1000 |
|
5 |
5 |
0.12345 |
Use this property to specify whether or not the column can contain null values.
For Microsoft Access data-types that contain string data (Text or Memo) you can specify whether or not a column can contain a zero-length string (""). This is not the same as a null value.
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