Finding which of an array of IDs has no record with a single query - mysql

Question

I'm generating prepared statements with PHP PDO to pull in information from two tables based on an array of IDs.
Then I realized that if an ID passed had no record I wouldn't know.
I'm locating records with
SELECT
r.`DEANumber`,
TRIM(r.`ActivityCode`) AS ActivityCode,
TRIM(r.`ActivitySubCode`) as ActivitySubCode,
// other fields...
a.Activity
FROM
`registrants` r,
`activities` a
WHERE r.`DEAnumber` IN ( ?,?,?,?,?,?,?,? )
AND a.Code = ActivityCode
AND a.Subcode = ActivitySubCode
But I am having trouble figuring out the negative join that says which of the IDs has no record.
If two tables were involved I think I could do it like this
SELECT
r.DEAnumber
FROM registrant r
LEFT JOIN registrant2 r2 ON r.DEAnumber = r2.DEAnumber
WHERE r2.DEAnumber IS NULL
But I'm stumped as to how to use the array of IDs here. Obviously I could iterate over the array and track which queries had not result but it seems like such a manual and wasteful way to go...

Answer 1

Obviously I could iterate over the array and track which queries had not result but it seems like such a manual and wasteful way to go.
What could be a real waste is spending time solving this non-existent "problem".
Yes, you could iterate. Either manually, or using a syntax sugar like array_diff() in PHP.
I suggest that instead of making your query more complex (means heavier to support) for little gain, you just move on.
As old man Knuth once said 'premature optimization is the root of all evil'.
The only thing I could think of a help from PDO is a fetch mode that will put IDs as keys for the returned array, and thus you'll be able to make it without [explicitly written] loop, like
$stmt->execute($ids);
$data = $stmt->fetchAll(PDO::FETCH_UNIQUE);
$notFound = array_diff($ids, array_keys($data));
Yet a manual loop would have taken only two extra lines, which is, honestly, not that a big deal to talk about.

Answer 2

You are on the right track - a left join that filters out matches will give you the missing joins. You just need to move all conditions on the left-joined table up into the join.
If you leave the conditions on the joined table in the where clause you effectively cause an inner join, because the where clause is executed on the rows after the join is made, which is too late if there was no join in the first place.
Change the query to use proper join syntax, specifying a left join, with the conditions on activity moved to the join'n on clause:
SELECT
r.DEANumber,
TRIM(r.ActivityCode) AS ActivityCode,
TRIM(r.ActivitySubCode) as ActivitySubCode,
// other fields...
a.Activity
FROM registrants r
LEFT JOIN activities a ON a.Code = ActivityCode
AND a.Subcode = ActivitySubCode
WHERE r.DEAnumber IN (?,?,?,?,?,?,?,?)
In your app code, if Activity is null then you know there was no activity for that id.
This won't affect performance much, other than to return (potentially) more rows.
To just select all registrants without activities:
select r.DEAnumber
from registrants r
left join activities a on a.Code = ActivityCode
and a.Subcode = ActivitySubCode
where r.`DEAnumber` IN ( ?,?,?,?,?,?,?,? )
and a.Code is null