Speeding up SQL loop - php

Question

I have a nested loop in my PHP code that is taking a long time and wondered of there was a way to speed this up:
$sql = "SELECT * FROM table_1";
$result = mysqli_query($sql);
while($row = mysqli_fetch_array($result)){
$sql2 = "
SELECT count(*)
FROM user_modules
WHERE
begin_ymdhi >= ".$date_from." AND
begin_ymdhi <= ".$date_to." AND
(
completed_ymdhi IS NULL OR
completed_ymdhi = ''
) AND
user_id = ".$row['user_id']." AND
task_id = ".$row['task_id']." AND
module_discon = 'N'
";
}
The outer query gets 1000 rows, the inner has to count across 10,000 rows - the run-time is around 115 seconds ! Is there any way to improve this method, either using a different technique or combined SQL query?

Answer 1

Don't use nested queries, combine them into a single query with a join:
SELECT t1.*, COUNT(u.user_id) ct
FROM table_1 t1
LEFT JOIN user_modules AS u ON u.user_id = t1.user_id AND u.task_id = t1.task_id
AND u.begin_ymdhi BETWEEN '$date_from' AND '$date_to'
AND u.module_discon = 'N'
GROUP BY t1.user_id, t1.task_id

Answer 2

Are the task_id's unique? if so, the most straight forward would be something like:
$sql2 = "
SELECT count(task_id) AS TaskCount
FROM user_modules
WHERE
begin_ymdhi >= ".$date_from." AND
begin_ymdhi <= ".$date_to." AND
(
completed_ymdhi IS NULL OR
completed_ymdhi = ''
) AND module_discon = 'N'
group by user_id
";
$result = mysqli_query($sql2);

Answer 3

SELECT user_modules.user_id, user_modules.task_id, count(*)
FROM user_modules LEFT JOIN table_1 USING (user_id, task_id)
WHERE
begin_ymdhi >= ".$date_from." AND
begin_ymdhi <= ".$date_to." AND
module_discon = 'N' AND
(
completed_ymdhi IS NULL OR
completed_ymdhi = ''
)
GROUP BY user_modules.user_id, user_modules.task_id
Append EXPLAIN before that entire SELECT statement (i.e EXPLAIN SELECT count(*)...) and MySQL will give you a run-down on what the select is doing.
Make sure the begin_ymdhi field is indexed properly. SHOW INDEX FROM table_2 to see.