Comment 1989 added to page sql-update.html of version 7.4

Author: westside <ethan@audio-crusade.com>
----
Just make Dominik's clearer if field2 is in table2

update table1 set field1 = table2.field2 from table2 where table1.id = table2.id;

This is needed especially if field1 and field2 have the same name.
----
Manual page: http://wwwmaster.postgresql.org/docs/7.4/interactive/sql-update.html
Reject:      http://wwwmaster.postgresql.org/admin/comments.php?action=reject&id=1989
Edit:        http://wwwmaster.postgresql.org/admin/comment-edit.php?id=1989
Delete:      http://wwwmaster.postgresql.org/admin/comments.php?action=delete&id=1989