How to select first row of the first table in an html page using jQuery?

$("table:first > tr:first")

or

$("table:first").find("tr:first")

or

$("table:first").children("tr:first")

or

$("table").eq(0).children("tr").eq(0)

So if I understand the followup question...

$("table:eq(1) tr:has(table:eq(2))")

translates to: get any tr's in the 2nd table's if the tr has a 3rd table

or

$("table").eq(1).children("tr:has(table:eq(2))")

Something you can use to select the nth row in the nth table:

$("table:eq(n) tr:eq(n)")

with n being the zero based index of the table or tr.

Example:

$("table:eq(2) tr:eq(4)")

gets the 5th row of the 3rd table.

@svinto's answer is definitely the shortest, quickest and easiest way to accomplish this. If you're really concerned with performance (e.g. selecting within an arbitrary complex for loop), this might most likely prove to be a tad faster:

$('tr').eq(0)

If you need not use only jQuery selectors and actually require the DOM <TR> element, you can use:

$('table')[0].rows[0]

Alternatively:

$('tr')[0]

Using jQuery's eq() method you can specify the index of the element you want to get.
This will select the first row of the second table found in the DOM

$('table:eq(1) tr:first')

Although not jQuery specific, I was introduced to dom selectors at this w3c selectors page. It's very detailed but full of intricate examples.

$("tr:first");