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。我不知道为什么它不起作用：

//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'"
);

//Deleting the post itself 有效，但 foreach 循环内的部分由于某种原因不起作用。为了调试，我在循环中添加了以下内容：

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

它按预期返回：

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

因此，获取的数据不是问题。我也试过了

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

而不是只使用cid，它也不起作用。回声它返回，正如预期的那样：

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

是的，我检查了comments 表，我要删除的id、pid 和cid 与正在回显的那些匹配。

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，您既可以解决此问题，也可以修复您现在遇到的安全问题（SQL 注入）...

这是您的代码转换为高效的预处理语句：

$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。处理这个问题的最好方法是使用外键。例如，让我们从 cmets 表的 pid 到帖子 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，您在上面的大代码块简化为：

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

当您删除评论时，约束（外键）将自动级联删除以删除 cmets（因为如果没有，则会有无效的约束）...

Answer 2

应该是

$this->db->exec(
"DELETE from comments 
WHERE cid = '$this->row[cid]'"
);

或者你也可以使用

$this->db->exec(
"DELETE from comments 
WHERE cid = '{$this->row['cid']}'"
);