Updating multiple rows in MySQL without a loop - php

Question

I'm trying to write up an email notification system for a job recruitment site I made and am currently looking at grouping a certain amount of jobs together before sending an email to the candidate
I have a table which I've called candidate_to_job which contains the candidate_id, job_id and an "emailed" boolean
What I'm struggling with is updating that table when a new job is posted and emails are sent out. So far when a new job is posted I run the following:
SELECT c2j.candidate_id, c2j.job_id, j.title
FROM " . DB_PREFIX . "candidate_to_job c2j
LEFT JOIN " . DB_PREFIX . "job j
ON (j.job_id = c2j.job_id)
WHERE c2j.emailed = 0
Then through PHP I group them all together so I have an array looking something like this:
$candidates = array(
1 => array(1,2,3),
2 => array(1,3),
3 => array(4,5,6)
);
With the array key being the candidate ID and the value an array of job IDs
What I want to do using that array - after the emails have been sent - is update the candidate_to_job table setting emailed to true, e.g candidate_id 1 would have emailed set to true for job_ids 1, 2 and 3
Is there a way I can do this in one query? I've looked at WHEN CASE statements but I don't think that applies in this case? I really don't want to run multiple queries per candidate because there could potentially be thousands!

Answer 1

You can run one UPDATE query per group provided that the group can share the same WHERE criteria and the same update values.
UPDATE tbl SET value = TRUE WHERE id IN(1,2,3,4,5,6);
UPDATE tbl SET value = FALSE WHERE id IN(7,8,9,10,11);
Or you can use the WHEN clause or even some IF clauses provided that the criteria are simple enough.
UPDATE tbl SET value = IF(id = 1) WHERE id IN(1,2);
UPDATE tbl
SET value = CASE
WHEN id IN (1,2,3,4,5,6) THEN TRUE
WHEN id IN (7,8,9,10,11) THEN FALSE
ELSE value
END
WHERE id IN (1,2,3,4,5,6,7,8,9,10,11);
Having possibly thousands of WHEN cases, may be a hassle to build/change, I'd go with the first option:
Flip the old key=>value array and keep all ids connected to a value:
foreach($list AS $id => $value) {
$list2[$value][] = $id;
}
Iterate through the value=>keys array and build UPDATE queries that can bulk update the value for all keys at once.

Answer 2

If you are using mysqli extension in PHP instead of mysql, you can also write mutliple statements seperated by ';' and send all of them in one query. This might result in slightly simpler code than a complex UPDATE with cases.
I dont think this costs much more performance than one single complex UPDATE statement.

Answer 3

Is this what you're after? You can do a join using from and where without having to dynamically generate SQL in PHP.
UPDATE " . DB_PREFIX . "candidate_to_job c2j
SET emailed = 1
FROM " . DB_PREFIX . "job j
WHERE j.job_id = c2j.job_id and
c2j.emailed = 0