PDO prepared statement seems to ignore HAVING clause - php

Question

I've included a DB-fiddle, and you can adjust the input parameter accordingly. This returns how I would expect it to, and differs from the results I am seeing in PDO.
I have the following minified table-view and query:
CREATE TABLE `tagged` {
`tag` SMALLINT(5) UNSIGNED NOT NULL
}
Table has an assortment of values, but you can use 1-10 for tags in the DB:
INSERT INTO tagged (tag) VALUES (1), (2), (3), (4), (5), (6), (7), (8), (9), (10)
query:
SELECT tagged.tag,
(#t := :tag),
#t AS temp_var,
(#t IS NULL OR FIND_IN_SET(tagged.tag, #t) > 0) AS is_match
FROM tagged
HAVING is_match = 1
LIMIT 150
This seems well and good when run in a client, command line, jdbc, etc. If I put in an input of '' or NULL, I get all results. Similarly an input of '1' yields only tags of 1, and an input of '1,4' would retrieve all tags with 1 or 4.
The way the query restricts these results is via the is_match = 1 in the HAVING clause. When run with PDO, the parameter seems to bind correctly but it completely ignores the condition in the clause:
Array
(
[0] => stdClass Object
(
[tag] => 3
[(#t := ?)] => 1,4
[temp_var] => 1,4
[is_match] => 0 ## should not have been returned
)
[1] => stdClass Object
(
[tag] => 4
[(#t := ?)] => 1,4
[temp_var] => 1,4
[is_match] => 1
)
PHP code used to run this (simplified):
$conn = /* pdo connection object */;
$stmt = $conn->prepare(DB::queryOf('test')); //uses our above query from a file
$stmt->bindValue(':tag', $args['tag'], PDO::PARAM_STR); //hardcode binding '1,4'
$stmt->execute(); //also tried plain #execute($args)
return $stmt->fetchAll(PDO::FETCH_OBJ);
Is there something I'm missing? I am binding a direct string parameter, and it seems the temporary variable is there and set correctly. Why is PDO returning the results for elements where is_match = 0?

Answer 1

I believe this behavior is dependent on the RDBMS being used.
In the absence of the GROUP BY clause, it seems that in some circumstances, the entire result can be considered as "one group". Because one row in the results satisfies the HAVING condition, all shall pass.
Additional reading:
Use of HAVING without GROUP BY in SQL queries
HAVING without GROUP BY
p.s. I don't think the > 0 is necessary.
I suppose I'd write your query something like this:
SELECT tag,
#t := '1,4' AS temp_var,
1 AS is_match
FROM tagged
WHERE #t IS NULL OR FIND_IN_SET(tag, #t)
LIMIT 150;