Tài liệu Retrieving New Identity Column Values doc

Retrieving New Identity Column Values
The ProductID column of the Products table is an identity column. In this section, you'll
see how to insert a new row into to Products table and retrieve the new value generated
by the database for the ProductID identity column.

N
ote You'll find a complete program named UsingIdentityColumn.cs in the ch11
directory that illustrates the use of the methods shown in this section. This program
listing is omitted from this book for brevity.
In the examples, assume you have a DataTable named productsDataTable that is
populated with the rows retrieved by the following SELECT statement:
SELECT
ProductID, ProductName, UnitPrice
FROM Products
ORDER BY ProductID
The following example sets the PrimaryKey property of productsDataTable:
productsDataTable.PrimaryKey =
new DataColumn[]
{
productsDataTable.Columns["ProductID"]
};
The next example sets the AllowDBNull, AutoIncrement, AutoIncrementSeed,
AutoIncrementStep, ReadOnly, and Unique properties for the ProductID DataColumn of
productsDataTable:
DataColumn productIDDataColumn =
productsDataTable.Columns["ProductID"];
productIDDataColumn.AllowDBNull = false;
productIDDataColumn.AutoIncrement = true;
productIDDataColumn.AutoIncrementSeed = -1;
productIDDataColumn.AutoIncrementStep = -1;

productIDDataColumn.ReadOnly = true;
productIDDataColumn.Unique = true;
Because of these settings, when you add a new DataRow to productsDataTable the
ProductID DataColumn of your new DataRow will initially have the value -1.
As in the earlier section, "Modifying Rows in a DataTable," you need to set your
DataAdapter object's InsertCommand, UpdateCommand, and DeleteCommand properties
with appropriate Command objects. The CommandText property of the Command object
used in the UpdateCommand property is as follows:
myUpdateCommand.CommandText =
"UPDATE Products " +
"SET " +
" ProductName = @NewProductName, " +
" UnitPrice = @NewUnitPrice " +
"WHERE ProductID = @OldProductID " +
"AND ProductName = @OldProductName " +
"AND UnitPrice = @OldUnitPrice";
The CommandText property of the Command object used in the DeleteCommand
property is as follows:
myDeleteCommand.CommandText =
"DELETE FROM Products " +
"WHERE ProductID = @OldProductID " +
"AND ProductName = @OldProductName " +
"AND UnitPrice = @OldUnitPrice";
Notice the CommandText of these two Command objects isn't substantially different
from those shown in the previous section
, except that it goes against the Products table
rather than the Customers table.
The real difference is in the CommandText of the Command object used in the
InsertCommand property, which must retrieve the ProductID value generated by the
database for the new row. To do this, you can use the following code that contains an

INSERT statement that adds a new row, along with a SELECT statement that retrieves
the ProductID value using a call to the SQL Server SCOPE_IDENTITY() function:
myInsertCommand.CommandText =
"INSERT INTO Products (" +
" ProductName, UnitPrice " +
") VALUES (" +
" @MyProductName, @MyUnitPrice" +
");" +
"SELECT @MyProductID = SCOPE_IDENTITY();";
myInsertCommand.Parameters.Add(
"@MyProductName", SqlDbType.NVarChar, 40, "ProductName");
myInsertCommand.Parameters.Add(
"@MyUnitPrice", SqlDbType.Money, 0, "UnitPrice");
myInsertCommand.Parameters.Add("@MyProductID", SqlDbType.Int,
0, "ProductID");
myInsertCommand.Parameters["@MyProductID"].Direction =
ParameterDirection.Output;
The SCOPE_IDENTITY() function returns the last inserted identity value into any table
performed within the current database session and stored procedure, trigger, function, or
batch. For example, calling SCOPE_IDENTITY() in the previous example returns the
last identity value inserted into the Products table, which is the ProductID of the new
row.

N
ote For details on the SCOPE_IDENTITY() function, refer to Chapter 4, "Introduction
to Transact-SQL Programming."
When you add a new DataRow to productsDataTable, the ProductID DataColumn of
your new DataRow will initially have the value -1. When you call the Update() method of
your SqlDataAdapter to push the new row to the database, the following steps occur:
1. Your new DataRow is pushed to the database using the INSERT statement set in

myInsertCommand, with the ProductID column of the Products table being set to a
new identity value generated by the database.
2. The ProductID identity value is retrieved by the SELECT statement set in
myInsertCommand.
3. The ProductID DataColumn in your DataRow is set to the retrieved identity value.
Feel free to examine, compile, and run the UsingIdentityColumn.cs program located in
the ch11 directory. This program performs the following high-level actions:
1. Retrieves rows from the Products table into a DataTable named
productsDataTable.
2. Adds a DataRow to productsDataTable.
3. Modifies the new DataRow.
4. Deletes the new DataRow.
When you run this program you'll notice the change in the ProductID DataColumn value
for a newly added DataRow from -1 to an actual value retrieved from the database.