Question

1. how to highlight the row when mouse is on,then de-highlight when mouse is out

2. how to update a specified row with new values?

3. how to get number of rows in table?

EDIT:
the one with best answer for q2 will be marked as answer for this post:)

Answer 1

One:

$('#mytable').find('tr').hover(function() {
    $(this).addClass('active');
}, function() {
    $(this).removeClass('active');
});

Along with this CSS:

#mytable tr.active td {
    background-color: #ccc;
}

Two:

You said update a “row” but all you can really update is cells, unless you want to create whole new cells.

$(cell).html('Contents');

Or:

var $cell = $('<td>').html('Contents');
$(row).html($cell);

Or if a table row has 3 cells, to update the first one:

$(row).find('td').eq(0).html('Contents');

Three:

$('#mytable').find('tr').length;