Question

I have a table of classes, and a table of subjects.

CLASS
|class_id|class_name|subject_id|date_time|
------------------------------------------
(imagine some rows here)

SUBJECT
|subject_id|subject_name|current_class_count|
---------------------------------------------
(imagine some rows here)

For indexing purposes I’d like to update the list of subjects every hour with the list of classes which have a date_time greater than right now.

I can do a select statement like this:

SELECT count(*) AS num, subject_id
FROM class
GROUP BY subject_id
where date_time > NOW()

and I will get something like

RESULT
|num|subject_id|
----------------
| 8 |        1 |
| 6 |        2 |
| 9 |        3 |
----------------

What’s the most efficient way to get the subject table updated with the current_class_count? I could do this with PHP by looping through and doing several update statements, but I think mySql should have an easier way.

Answer 1

Edit: How about this:

UPDATE SUBJECT
LEFT JOIN (
SELECT count(*) AS num, subject_id
FROM class
GROUP BY subject_id
where date_time > NOW()) AS t ON SUBJECT.subject_id = t.subject_id
SET SUBJECT.current_class_count = coalesce( t.num, 0 )

As long as I’ve typed it right, basically you should be able to run this once an hour and it will update your SUBJECT table.

Joining a table in an UPDATE statement is a bit different in MySQL compared to Microsoft SQL. Here is a link about it:

http://blog.ookamikun.com/2008/03/mysql-update-with-join.html