Doubts in '--update-mode allowinsert' argument

[sam liu <li...@gmail.com>]

In Sqoop User Guide (v1.4.1-incubating), it says 'you may also specify the
--update-mode argument with allowinsert mode if you want to update rows if
they exist in the database already or insert rows if they do not exist yet'.

However, I have some questions:
1: How to determine if a row is existing or not? By a specific column or
key?
  (Actually I failed to execute a command with --update-mode argument and
--update-key argument together)

2: If I did not specific a column to identify the row, I do not think the
following result I got is as expected, because I want to update the first
row, but sqoop insert a similar row into my table. Any idea?

+------+-------------+-------+-------------+---------+----------------------+
| id   | widget_name | price | design_date | version | design_comment
|
+------+-------------+-------+-------------+---------+----------------------+
|    1 | sprocket    |  0.25 | 2010-02-10  |       3 | Connects two gizmos
|
|    1 | sprocket    |  0.25 | 2010-02-10  |       1 | Connects two gizmos
|
|    2 | gizmo       |  4.00 | 2009-11-30  |       4 | NULL
|
|    3 | gadget      | 99.99 | 1983-08-29  |      13 | Our flagship product
|
+------+-------------+-------+-------------+---------+----------------------+