How does Update Command of Data Adapter object work?

The Update method of the DataAdapter is called to resolve changes from a DataSet back to the data source. The Update method, like the Fill method, takes as arguments an instance of a DataSet, and an optional DataTable object or DataTable name. The DataSet instance is the DataSet that contains the changes that have been made, and the DataTable identifies the table from which to retrieve the changes.
When you call the Update method, the DataAdapter analyzes the changes that have been made and executes the appropriate command (INSERT, UPDATE, or DELETE). When the DataAdapter encounters a change to a DataRow, it uses the InsertCommand, UpdateCommand, or DeleteCommand to process the change. This allows you to maximize the performance of your ADO.NET application by specifying command syntax at design-time and, where possible, through the use of stored procedures. You must explicitly set the commands before calling Update. If Update is called and the appropriate command does not exist for a particular update (for example, no DeleteCommand for deleted rows), an exception will be thrown.
Command parameters can be used to specify input and output values for an SQL statement or stored procedure for each modified row in a DataSet.
If your DataTable maps to or is generated from a single database table, you can take advantage of the CommandBuilder object to automatically generate the DeleteCommand, InsertCommand, and UpdateCommand of the DataAdapter.
The Update method will resolve your changes back to the data source, however other clients may have modified data at the data source since the last time you filled the DataSet. To refresh your DataSet with current data, use the DataAdapter and Fill the DataSet again. New rows will be added to the table, and updated information will be incorporated into existing rows. The Fill method determines whether a new row will be added or an existing row will be updated by examining the primary key values of the rows in the DataSet and the rows returned by the SelectCommand. If the Fill method encounters a primary key value for a row in the DataSet that matches a primary key value from a row in the results returned by the SelectCommand, it updates the existing row with the information from the row returned by the SelectCommand and sets the RowState of the existing row to Unchanged. If a row returned by the SelectCommand has a primary key value that does not match any of the primary key values of the rows in the DataSet, the Fill method adds a new row with a RowState of Unchanged.
Note If the SelectCommand returns the results of an OUTER JOIN, the DataAdapter will not set a PrimaryKey value for the resulting DataTable. You will need to define the PrimaryKey yourself to ensure that duplicate rows are resolved correctly.