PHP和MySQL，这个函数有什么问题？

这个函数不起作用，我找不到问题所在，问题就在查询的某个地方

function get_category_posts($category, $page, $per_page) {
$start = (int)($page - 1) * $per_page;
$per_page = (int)$per_page;

$sql = "SELECT
    `posts`.`post_id` AS `id`,
    `posts`.`post_title` AS `title`,
    `posts`.`post_category` AS `category`,
    `posts`.`post_body` AS `preview`,
    `posts`.`post_user` AS `user`,
    DATE_FORMAT(`posts`.`post_date`, '%Y-%m-%d %H:%i:%s') AS `date`,
    `comments`.`total_comments`,
    DATE_FORMAT(`comments`.`last_comment`, '%Y-%m-%d %H:%i:%s') AS `last_comment`
FROM `posts`
WHERE `category`='".$category."'
LEFT JOIN (
    SELECT
        `post_id`,
        COUNT(`comment_id`) AS `total_comments`,
        MAX(`comment_date`) AS `last_comment`
    FROM `comments`
    GROUP BY `post_id`
) AS `comments`
ON `posts`.`post_id` = `comments`.`post_id`
ORDER BY `posts`.`post_date` DESC
LIMIT {$start}, {$per_page}";

$posts = mysql_query($sql);
//die(mysql_error());

$rows = array();
while (($row = mysql_fetch_assoc($posts)) !== false) {
    $rows[] = array(
        'id' => $row['id'],
        'title' => html_entity_decode($row['title']),
        'category' => html_entity_decode($row['category']),
        'preview' => html_entity_decode($row['preview']),
        'user' => html_entity_decode($row['user']),
        'date' => $row['date'],
        'total_comments' => ($row['total_comments'] === null) ? 0 : $row['total_comments'],
        'last_comment' => ($row['last_comment'] === null) ? 'aldrig' : $row['last_comment']
        );
}

return $rows;

} 函数在没有WHEREcategory='.$category'的情况下可以正常工作，但是，它返回

您的SQL语法有错误；查看与MySQL服务器版本相对应的手册，了解第12行中使用“左连接选择post\u id，COUNTcomment\u id作为total\u comments”的正确语法

---

WHERE子句必须位于表联接之后、ORDER BY和LIMIT之前

SELECT <column list>
FROM <table>
JOIN <table> USING (<column>)
WHERE <condition>
OR <condition>
AND <condition>
GROUP BY <column list>
HAVING <expression>

---

连接必须在WHERE条件之前出现

$sql = "SELECT
    `posts`.`post_id` AS `id`,
    `posts`.`post_title` AS `title`,
    `posts`.`post_category` AS `category`,
    `posts`.`post_body` AS `preview`,
    `posts`.`post_user` AS `user`,
    DATE_FORMAT(`posts`.`post_date`, '%Y-%m-%d %H:%i:%s') AS `date`,
    `comments`.`total_comments`,
    DATE_FORMAT(`comments`.`last_comment`, '%Y-%m-%d %H:%i:%s') AS `last_comment`
FROM `posts`
LEFT JOIN (
    SELECT
        `post_id`,
        COUNT(`comment_id`) AS `total_comments`,
        MAX(`comment_date`) AS `last_comment`
    FROM `comments`
    GROUP BY `post_id`
) AS `comments`
ON `posts`.`post_id` = `comments`.`post_id`
WHERE `posts`.`category`='".$category."'
ORDER BY `posts`.`post_date` DESC
LIMIT {$start}, {$per_page}";

---

如果我把它放在连接之后，我会得到这样一个结果：“where子句”中的未知列“category”，这是因为category不在表中-它只是一个列别名，请尝试使用posts.post_category代替headi得到这个结果：“where子句”中的未知列“category”。