A unique key is a column or set of columns that uniquely identify a row; in other words, each value in a unique-key column is unique. A unique key constraint prevents duplicate (non-unique) and null values from being entered. The only difference between a unique key and a primary key is that you can define multiple unique keys per table.
Case Sensitive - specifies that the data values will be case sensitive
Message - specifies the messages for various error conditions
Message on insert of a Unique value which is not unique
The message that will be displayed to a user attempting to insert a row with a field value which already exists in the table. A default message is provided, and can be changed only when the key is created.
Message on delete of a Unique value also in a referencing Foreign Key
The message that will be displayed to a user attempting to delete a row with a field value which is referenced by a Foreign Key in another table. A default message is provided, and can be changed only when the key is created.
Message on update of a Unique value also in referencing Foreign Key
The message that will be displayed to a user attempting to update a row with a field value which is referenced by a Foreign Key in another table. A default message is provided, and can be changed only when the key is created.
After the unique key definition is complete, the unique key is displayed in the Data Designer window. Once the key is selected, the "Column(s)" panel displays the column(s) that the key is based upon and the sorting order. If the unique key is based on a TEXT column, the text length is also provided. The unique value and referenced table enforcement "Messages" are also listed when the unique key is selected.
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