php - 无法在循环中从 MySQL 表中删除多行

Question

我正在尝试使用 php 的 PDO 对象从 mySQL 表中删除与一对条件匹配的所有行。我不明白为什么它不起作用:

//This deletes all comments for a given post;
//Querying database for existent comments on that post;
$this->query = $this->db->query(
"SELECT cid 
FROM comments
WHERE id = '$html_data->id' AND pid = '$html_data->pid'"
);
//Fetching results into an array;
$this->rows = $this->query->fetchAll(PDO::FETCH_ASSOC);
//Deleting each comment on that post;
foreach ($this->rows as $this->row) {
    $this->db->exec(
    "DELETE from comments 
    WHERE cid = '$this->row['cid']'"
    );
};
//Deleting the post itself;
$this->db->exec(
"DELETE from posts 
WHERE id = '$html_data->id' AND pid = '$html_data->pid'"
);

//删除帖子本身可以工作，但foreach循环内的部分由于某种原因不起作用。为了调试，我在循环内添加了以下内容:

echo "WHERE cid = '{$this->row['cid']}'";

它按预期返回:

WHERE cid = '1'
WHERE cid = '2'

所以正在获取的数据不是问题。我也尝试过

WHERE id = '$html_data->id' AND pid = '$html_data->pid' AND cid = '$this->row['cid']'"

而不是仅使用cid，它也不起作用。正如预期的那样，它会返回:

WHERE id = '1' AND pid = '1' AND cid = '1'
WHERE id = '1' AND pid = '1' AND cid = '2'

是的，我检查了我想要删除的 comments 表、id、pid 和 cid匹配那些正在回显的内容。

Answer 1

因此，解决此问题的更好方法是使用准备好的语句并从查询中获取变量。这样，您既可以解决这个问题，也可以解决您现在遇到的安全问题(SQL 注入(inject))...

这是转换为高效准备语句的代码:

$stmt= $this->db->prepare(
    "SELECT cid 
    FROM comments
    WHERE id = ? AND pid = ?"
);
$this->query = $stmt->execute(array($html_data->id, $html_data->pid));

$this->rows = $this->query->fetchAll(PDO::FETCH_ASSOC);

$deleteStmt = $this->db->prepare(
    "DELETE from comments 
    WHERE cid = ?"
);
foreach ($this->rows as $row) {
    $deleteStmt->execute(array($row['cid']));
};

//Deleting the post itself;
$stmt = $this->db->prepare(
    "DELETE FROM posts
    WHERE id = ? AND pid = ?"
);
$stmt->execute(array($html_data->id, $html_data->pid));

但是，您可以进一步清理它。处理这个问题的最佳方法是使用外键。例如，让我们从评论表的 pid 到 posts id 字段创建一个外键。

CREATE TABLE posts (
    id   INT,
    name VARCHAR(35),
    PRIMARY KEY (`id`)
) ENGINE = InnoDB;

CREATE TABLE comments (
    id   INT,
    pid  INT,
    name VARCHAR(35),
    PRIMARY KEY (`id`),
    CONSTRAINT `posts_id`
    FOREIGN KEY `posts_id` (`pid`)
    REFERENCES `posts` (`id`)
    ON DELETE CASCADE
    ON UPDATE CASCADE
) ENGINE = InnoDB

这里的美妙之处在于你的大代码块简化为这样:

$stmt = $this->db->prepare(
    "DELETE FROM posts
    WHERE id = ? AND pid = ?"
);
$stmt->execute(array($html_data->id, $html_data->pid));

当你删除评论时，约束(外键)会自动级联删除来删除评论(因为如果不这样做，就会有一个无效的约束)...