如何在MySQL中增加列值的个数?[副本]

我试图在用户下载了一个文件后添加一个数字,但我所做的一切都不起作用。

    $download = "INSERT INTO table (user_id,post_id)
    VALUES('$username','$postname')";

    $query = mysqli_query($con,$download);

    $post_data= "UPDATE posts SET downloaded = downloaded+1 WHERE post_id='$postname' AND user_id= '$username'";

    /*
    * The problem is here
    */
    $update_post_data = mysqli_query($con,$post_data);

我尝试过其他方法,比如“downloaded”=“downloaded”+1和“downloaded”+1,但都不管用。我该怎么办?