Count/Check first instances of entries in mysql table - php

Question

id | user_id | date_tracked
---------------------------------------------
1001 | 1 | 10-10-2013
1002 | 2 | 10-10-2013
1003 | 3 | 10-10-2013
1004 | 1 | 10-11-2013
1005 | 2 | 10-12-2013
I have a table similar to this, which tracks a user and a date. I need to find how many first-time entries for users occurred on a selected date. This table is going to hold a significant amount of data, which is why I'd like to rely on a query to process this instead of a bunch of PHP loops.
i.e. on 10-11-2013, user 1 visited but not their first time, so return 0
on 10-10-2013, user 1 and 2 visited for their first time, so return 2.
Obviously, using a simple query can count the number of entries on a specified date, but what methodology will allow me to only count if the user_id is not present on a row prior.
The table is ordered by date, meaning that a more recent date should never have a smaller id than an older date.
Any ideas?? Thanks!

Answer 1

Finding all "first time"-date, by user:
SELECT user_id, MIN(date_tracked) AS first_date
FROM table
GROUP BY user_id
Counting "first time", for each date:
SELECT t.first_date, COUNT(*) AS nb
FROM (SELECT user_id, MIN(date_tracked) AS first_date
FROM table
GROUP BY user_id) t
GROUP BY t.first_date

Answer 2

In response to Filipe,
Maybe this would be more suitable?
SELECT COUNT(*)
FROM table t1
WHERE t1.date ='2013-8-27'
AND NOT EXISTS (
SELECT 1
FROM table t2
WHERE t2.user_id = t1.user_id
AND t2.id < t1.id
);

Answer 3

SELECT user_id, MIN(date_tracked)
FROM table_name
GROUP BY user_id

Answer 4

By your tags I assume you want your answer in php as well.
Just one query: shouldn't date 10-10-2013 return 3?
Anyway you can try this:
<?php
$dbConnect = #mysqli_connect($host, $user, $pass)or die();
#mysqli_select_db($dbConnect, $dbname) or die();
$query = "SELECT id FROM tablename WHERE date_tracked = '$searchDate' AND (SELECT COUNT(id) FROM tablename WHERE date_tracked < '$searchDate')=0";
$queryResult = #mysqli_query($dbConnect, $query) or die();
$rowCount = mysqli_num_rows($queryResult);
echo $rowCount . "<br/>";
mysqli_close($dbConnect);
?>

Answer 5

For counting as one, if the user visits multiple times in the first day, you can do it in two ways:
SELECT COUNT(distinct user_id)
FROM user t1
WHERE t1.date_tracked = '2013-10-12'
AND NOT EXISTS (
SELECT 1
FROM user t2
WHERE t2.user_id = t1.user_id
AND t2.date_tracked < t1.date_tracked
);
SELECT COUNT(user_id)
FROM user t1
WHERE t1.date_tracked ='2013-10-12'
AND NOT EXISTS (
SELECT 1
FROM user t2
WHERE t2.user_id = t1.user_id
AND t2.id < t1.id
);
I think i prefer the second, as it is much more cleaner by comparing ID's and not having to do the distinct.
sqlfiddle demo