在php中选择WHERE子句

Question

I'm trying to make a quiz application and I need to retrieve "n" number of questions from a database. I also need to filter the result with categories or difficulties filters.

I don't have any problem if the user make a category choice (for example) :

$size = $_POST['param0'];
$category = $_POST['param1'];

$stmt = $db->prepare("SELECT *
                      FROM questions_fr
                      WHERE categories = :categories
                      LIMIT 0, $size");

$stmt->bindparam(":category", $category);
$stmt->execute();

But what if he wants all the categories ?

Can I do something like that to make only one query in my php file ?

$stmt = $db->prepare("SELECT *
                      FROM questions_fr
                      WHERE categories = * (here, select all the categories)
                      LIMIT 0, $size");

Or should I do something like this ?

(pseudocode)
if ($category != "all_categories")
{
    $stmt = $db->prepare("SELECT *
                          FROM questions_fr
                          WHERE categories = :categories
                          LIMIT 0, $size");
}
else if ($category == "all_categories")
{
    $stmt = $db->prepare("SELECT *
                          FROM questions_fr
                          LIMIT 0, $size");
}

Answer 1

If they select 'all categories' set the category variable to a wildcard and use the LIKE clause.

if ($category == "all_categories") {
    $category = '%';
}

...

SELECT *
FROM questions_fr
WHERE categories LIKE :categories
LIMIT 0, $size