Defining Primary Keys

A database table commonly has a column or group of columns that uniquely identifies each row in the table. This identifying column or group of columns is called the primary key.

When you identify a single DataColumn as the PrimaryKey for a DataTable, the table automatically sets the AllowDBNull property of the column to false and the Unique property to true. For multiple-column primary keys, only the AllowDBNull property is automatically set to false.

The PrimaryKey property of a DataTable receives as its value an array of one or more DataColumn objects, as shown in the following examples. The first example defines a single column as the primary key.

workTable.PrimaryKey = New DataColumn() {workTable.Columns("CustID")}  
' Or  
Dim columns(1) As DataColumn  
columns(0) = workTable.Columns("CustID")  
workTable.PrimaryKey = columns  
workTable.PrimaryKey = new DataColumn[] {workTable.Columns["CustID"]};  
// Or  
DataColumn[] columns = new DataColumn[1];  
columns[0] = workTable.Columns["CustID"];  
workTable.PrimaryKey = columns;  

The following example defines two columns as a primary key.

workTable.PrimaryKey = New DataColumn() {workTable.Columns("CustLName"), _  
' Or  
Dim keyColumn(2) As DataColumn  
keyColumn(0) = workTable.Columns("CustLName")  
keyColumn(1) = workTable.Columns("CustFName")  
workTable.PrimaryKey = keyColumn  
workTable.PrimaryKey = new DataColumn[] {workTable.Columns["CustLName"],
// Or  
DataColumn[] keyColumn = new DataColumn[2];  
keyColumn[0] = workTable.Columns["CustLName"];  
keyColumn[1] = workTable.Columns["CustFName"];  
workTable.PrimaryKey = keyColumn;  

See also