来自中间表的MySQL计数

Question

我试图在包含两个ID的中介表中找到最常用的类别，我正在运行它来查找针对类别的所有帖子数。

$data = $conn->query('SELECT * FROM category WHERE category_name ORDER BY category_name');

foreach($data as $row) {

$sql = "SELECT COUNT(p_id) FROM p_categories c,category ca WHERE c.category_id = ca.category_id AND category_name = :category_name";
$q = $conn->prepare($sql);
$q->execute(array(':category_name' => $row['category_name']));
$catco = $q->fetch();

echo '<p>'.$row['category_name'].' ('.$catco[0].')</p>';
}

这将返回所有类别，但是我想将其限制为仅具有最多p_id的前四个类别。 我尝试将计数查询限制为4，但这没有用。

表格如下：

帖子：p_id，p_name

p_categories：p_id，category_id

类别：category_id，category_name

Answer 1

您可以在单个查询中执行此操作

$sql = "SELECT p_categories.category_id, COUNT(*) AS cnt, category.category_name
        FROM p_categories 
        LEFT JOIN category
        ON p_categories.category_id = category.id 
        GROUP BY `category_id` 
        ORDER BY cnt 
        DESC LIMIT 4"

$data = $conn->query($sql); 
foreach($data as $row) {
    echo '<p>'.$row['category_name'].' ('.$row['cnt'] .')</p>';
}