Mysql - sql query to get next class based on date - mysql

Question

I have the following data in my webinar_timing table in mysql database
start_time and end_time are of type datetime
id | webinar_id | start_time | end_time
-------------------------------------------------------------------
1 | 5 | 3/18/2015 6:00:00 PM | 3/18/2015 7:00:00 PM
2 | 5 | 3/19/2015 6:00:00 PM | 3/19/2015 7:00:00 PM
3 | 5 | 3/20/2015 6:00:00 PM | 3/20/2015 7:00:00 PM
4 | 5 | 3/21/2015 6:00:00 PM | 3/21/2015 7:00:00 PM
5 | 5 | 3/22/2015 6:00:00 PM | 3/22/2015 7:00:00 PM
6 | 11 | 3/20/2015 8:00:00 PM | 3/20/2015 9:00:00 PM
7 | 11 | 3/21/2015 8:00:00 PM | 3/21/2015 9:00:00 PM
8 | 11 | 3/22/2015 8:00:00 PM | 3/22/2015 9:00:00 PM
9 | 22 | 3/25/2015 8:00:00 PM | 3/25/2015 9:00:00 PM
10 | 22 | 3/27/2015 8:00:00 PM | 3/27/2015 9:00:00 PM
11 | 22 | 3/29/2015 8:00:00 PM | 3/27/2015 9:00:00 PM
Basically, for each webinar, I want the total occurences and number of classes completed or remaining AND the NEXT upcoming class
Egs: When I run this query say at 3/21/2015 at 4:00 PM - this is the result I am expecting
webinar_id | total | Classes Completed | Next Class
----------------------------------------------------------
5 | 5 | 3 | 3/21/2015 6:00:00 PM
11 | 3 | 1 | 3/21/2015 8:00:00 PM
22 | 3 | 0 | 3/25/2015 8:00:00 PM
OR
webinar_id | total | Classes Remaining | Next Class
----------------------------------------------------------
5 | 5 | 2 | 3/21/2015 6:00:00 PM
11 | 3 | 2 | 3/21/2015 8:00:00 PM
22 | 3 | 3 | 3/25/2015 8:00:00 PM
Based on a previous question - a fellow SO Peter assisted with the following
select webinar_id, count(*) AS total,
SUM(IF(end_time<NOW(), 1, 0)) AS completed,
SUM(IF(start_time>=NOW(), 1, 0)) AS remaining
from webinar_times
group by webinar_id;
SQL Fiddle
http://sqlfiddle.com/#!9/c4e71/1
Any help will be appreciated
Thanks in advance

Answer 1

Something like:
select webinar_id
, count(*) AS total
, count(case when end_time<NOW() then 1 end) as completed
, (select count(1)
from webinar_times y
where x.webinar_id = y.webinar_id
and y.start_time > NOW()) as remaining
, min(case when x.start_time > NOW() then x.start_time end) as next_class
from webinar_times x
group by webinar_id;
should do
EDIT: realized that the sub-select is un-necessary:
select webinar_id
, count(*) AS total
, count(case when end_time<NOW() then 1 end) as completed
, count(case when start_time>NOW() then 1 end) as remaining
, min(case when x.start_time > NOW() then x.start_time end) as next_class
from webinar_times x
group by webinar_id;

Answer 2

You can make an outer join between two grouped queries, e.g. one that counts the total number of webinars and another that both counts the remaining webinars and obtains the start time of the next one:
SELECT * FROM (
SELECT webinar_id, COUNT(*) total
FROM webinar_times
GROUP BY webinar_id
) totals NATURAL LEFT JOIN (
SELECT webinar_id, COUNT(*) remaining, MIN(start_time) next
FROM webinar_times
WHERE start_time > NOW()
GROUP BY webinar_id
) future
See it on sqlfiddle:
+------------+-------+-----------+-------------------------+
| webinar_id | total | remaining | next |
+------------+-------+-----------+-------------------------+
| 6 | 5 | 1 | March, 22 2015 06:00:00 |
| 11 | 3 | 1 | March, 22 2015 07:00:00 |
| 22 | 3 | 3 | March, 25 2015 07:00:00 |
+------------+-------+-----------+-------------------------+
A composite index defined over (webinar_id, start_time) would benefit this query, and avoids the full table scans that the approach outlined in your question would otherwise require.

Answer 3

Consider the following example and this will give you what you need
mysql> create table test (id int, webinar_id int, start_time datetime);
Query OK, 0 rows affected (0.16 sec)
mysql> insert into test values (1,5,'2015-03-18 18:00:00'),
(2,5,'2015-03-19 18:00:00'),
(3,5,'2015-03-20 18:00:00'),
(4,5,'2015-03-21 18:00:00'),
(5,5,'2015-03-21 18:00:00'),
(6,11,'2015-03-20 20:00:00'),
(7,11,'2015-03-21 20:00:00'),
(8,11,'2015-03-22 20:00:00'),
(9,22,'2015-03-25 20:00:00'),
(10,22,'2015-03-27 20:00:00'),
(11,22,'2015-03-29 20:00:00');
Query OK, 11 rows affected (0.05 sec)
Records: 11 Duplicates: 0 Warnings: 0
mysql> select * from test ;
+------+------------+---------------------+
| id | webinar_id | start_time |
+------+------------+---------------------+
| 1 | 5 | 2015-03-18 18:00:00 |
| 2 | 5 | 2015-03-19 18:00:00 |
| 3 | 5 | 2015-03-20 18:00:00 |
| 4 | 5 | 2015-03-21 18:00:00 |
| 5 | 5 | 2015-03-21 18:00:00 |
| 6 | 11 | 2015-03-20 20:00:00 |
| 7 | 11 | 2015-03-21 20:00:00 |
| 8 | 11 | 2015-03-22 20:00:00 |
| 9 | 22 | 2015-03-25 20:00:00 |
| 10 | 22 | 2015-03-27 20:00:00 |
| 11 | 22 | 2015-03-29 20:00:00 |
+------+------------+---------------------+
11 rows in set (0.00 sec)
select
t.webinar_id,
count(*) as total,
sum( case when t.start_time < now() then 1 else 0 end) as completed ,
sum( case when t.start_time > now() then 1 else 0 end) as remaining,
t1.next_date from test t
join (
select
webinar_id,
min(start_time) as next_date
from test where start_time > now()
group by webinar_id
)t1 on t.webinar_id= t1.webinar_id
group by t.webinar_id;
+------------+-------+-----------+-----------+---------------------+
| webinar_id | total | completed | remaining | next_date |
+------------+-------+-----------+-----------+---------------------+
| 5 | 5 | 3 | 2 | 2015-03-21 18:00:00 |
| 11 | 3 | 1 | 2 | 2015-03-21 20:00:00 |
| 22 | 3 | 0 | 3 | 2015-03-25 20:00:00 |
+------------+-------+-----------+-----------+---------------------+
3 rows in set (0.00 sec)