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cn.Open(); cm.ExecuteNonQuery(); cn.Close(); tx.Complete(); } } The method to create a new order item follows (also in XAction.cs in the App_Code directory of the Web12 project). public bool AddOrderItem(int orderId, int itemId, int quantity) { using (TransactionScope tx = new TransactionScope(TransactionScopeOption.Required)) { if (UpdateQuantity(itemId, quantity)) { string sql = "INSERT INTO OrderItem(OrderID, InventoryID, Quantity) " + "VALUES (@OrderID, @InventoryID, @Quantity)"; SqlConnection cn = new SqlConnection(connStr); SqlCommand cm = new SqlCommand(sql, cn); cm.Parameters.Add(new SqlParameter("@OrderID", SqlDbType.Int)).Value = orderId; cm.Parameters.Add(new SqlParameter("@InventoryID", SqlDbType.Int)).Value = itemId; cm.Parameters.Add(new SqlParameter("@Quantity", SqlDbType.Int)).Value = quantity; cn.Open(); cm.ExecuteNonQuery(); cn.Close(); tx.Complete(); return true; } else return false; } } Notice the first thing that this method does after enlisting in the transaction is to call the UpdateQuantity method (listed previously). If this method fails, it means you re out of stock on the item being ordered, and so this method will not create the order item. It also does not call Complete in this case, which in effect dooms the entire transaction. UpdateQuantity is what

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Let s add another employee by the name of Jack, who works for John, and commit the data: benchmark@ORA10G> insert into emp_table_with_ref 2 select emp_ref_type( 3, 'Jack', ref(e)) 3 from emp_table_with_ref e 4 where emp_no = 2; 1 row created. benchmark@ORA10G> commit; Commit complete. A simple select * on the table emp_table_with_ref shows the following rows: benchmark@ORA10G> select * from emp_table_with_ref; EMP_NO NAME MANAGER ------- ----- ---------------------------------------1 Larry 2 John 0000220208529852ACD8C148BE9BE6A27D01FDA6 5EF5DECA545DC24435820B9DB4F9A2BCD0 3 Jack 00002202089B3B1CBC855A4038B15A4BE9788105 E4F5DECA545DC24435820B9DB4F9A2BCD0 The value shown in the manager column of type emp_ref_type is the object ID of the reference value. The following select shows how to dereference the reference to get the underlying values by using the function deref() on the result of the value() function. The value() function takes as its argument a table alias associated with a row of an object table and returns object instances stored in the object table. The function deref() returns the object reference of its argument, which must return a ref to an object. The following query selects the manager of the employee Jack: benchmark@ORA10G> select value(e).name Name, deref(value(e).manager) Manager 2 from emp_table_with_ref e 3 where e.name = 'Jack'; NAME MANAGER(EMP_NO, NAME, MANAGER) ----- ---------------------------------------Jack EMP_REF_TYPE(2, 'John', 0000220208B40B43 4961904E44AF83AC8FEE99EECEE983C5A859704C F6889D35D3862EDB06) In the next section, we discuss an important issue related to dangling references (references that point to a row that has been deleted) and how to overcome it.

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The schema created in the preceding section can result in what are known as dangling references. A dangling reference is a pointer that points to a row that has been deleted. Having dangling references can thus result in data integrity issues with some of the references in your schema pointing to nonexistent rows. For example, we can delete the record corresponding to John, even though the employee under him (Jack) still exists in the table: benchmark@ORA10G> delete from emp_table_with_ref e 2 where e.name = 'John'; 1 row deleted. The following query confirms that the reference in the row corresponding to Jack is now gone: benchmark@ORA10G> select value(e).name Name, deref(value(e).manager) Manager 2 from emp_table_with_ref e 3 where e.name = 'Jack'; NAME MANAGER(EMP_NO, NAME, MANAGER) ----- ---------------------------------------Jack In fact, Oracle provides a way to select dangling references by using the is dangling (or the opposite version, is not dangling) predicate in where clause as follows: benchmark@ORA10G> select value(e).name Name, deref(value(e).manager) Manager 2 from emp_table_with_ref e 3 where e.name = 'Jack' 4 and value(e).manager is not dangling; no rows selected However, the best thing to do if you use references is to create a referential integrity constraint (a foreign key) and avoid dangling references in your application. To illustrate this, we drop the table emp_table_with_ref and re-create it as follows (notice the references clause, shown in bold, that creates the constraint): benchmark@ORA10G> create table emp_table_with_ref of emp_ref_type 2 ( manager references emp_table_with_ref ); Table created. If we now repopulate the table with the same data and try to delete the row corresponding to John, we get the following error: benchmark@ORA10G> delete from emp_table_with_ref e 2 where e.name = 'John'; delete from emp_table_with_ref e *

The OleDb and ODBC data providers are provided for compatibility with earlier database access technologies. All ADO.NET connection and command classes have the data provider name as the prefix to their class name, as in OdbcConnection and OdbcCommand or OleDbConnection and OleDbCommand.