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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

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"], 
                                         workTable.Columns["CustFName"]};

// Or

DataColumn[] keyColumn = new DataColumn[2];
keyColumn[0] = workTable.Columns["CustLName"];
keyColumn[1] = workTable.Columns["CustFName"];
workTable.PrimaryKey = keyColumn;
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