How to check if a row exists which contains certain content in MySQL

Question

I am using the below code to insert ratings for specific songs on my application.

I am recording the songID, the rating given to the song and the userID who has voted on the song.

What I want to do is to prevent a user from voting if they have 'already' voted on a specific song. Therefore I need to be able to check if a row exists in the table before insertion.

So... If userID = 1, songID = 1 and rating = 4. This should insert fine.

If subsequently, an insertion attempt is made for userID=1, songID=1, rating=*, it should fail to insert

However if the user is voting on a different song... that should be allowed and the insertion should happen.

Any ideas how I would go about this?

//Add rating to database

if(!empty($_POST['rating']) && isset($_POST))
{
    //make variables safe to insert
  $rating = mysql_real_escape_string($_POST['rating']);
  $songid = mysql_real_escape_string($_POST['song_id']);

    //query to insert data into table
    $sql = "
        INSERT INTO wp_song_ratings
        SET
        songid = '$songid',
        rating = '$rating',
        userid = '$user_id'";
    $result = mysql_query($sql);
    if(!$result)
    {
        echo "Failed to insert record";
    }
    else
    {
        echo "Record inserted successfully";
    }
}

Answer 1

Add a UNIQUE KEY for userID and songID . If you don't want them to change their rating you shouldn't allow it from the front-end, but still make the check on the backend. It will fail if a UNIQUE KEY is in place.

ALTER TABLE `wp_song_ratings` ADD UNIQUE KEY `user_song_key` (`userID`, `songID`)

Answer 2

听起来您需要在mysql中使用存储过程-此处提供更多信息http://dev.mysql.com/doc/refman/5.0/en/create-procedure.html

Answer 3

You can first do a SELECT statement:

if(!empty($_POST['rating']) && isset($_POST))
{
    //make variables safe to insert
  $rating = mysql_real_escape_string($_POST['rating']);
  $songid = mysql_real_escape_string($_POST['song_id']);

  $select_sql = "SELECT COUNT(*) WHERE songid='$songid' AND userid='$user_id'";
  $select_result = mysql_query($select_sql);

  if ( mysql_num_rows($select_result) > 0 )
  {
    /* already voted! */
  }
  else
  {
    //query to insert data into table
    $sql = "
        INSERT INTO wp_song_ratings
        SET
        songid = '$songid',
        rating = '$rating',
        userid = '$user_id'";
    $result = mysql_query($sql);
    if(!$result)
    {
        echo "Failed to insert record";
    }
    else
    {
        echo "Record inserted successfully";
    }
  }
}

Of course, there are also other ways to implement such things (like stored procedures).

Answer 4

Add the following WHERE clause to your sql statement

WHERE NOT EXISTS (SELECT 1 FROM wp_song_ratings WHERE songid= '$songid' and userid = '$user_id')

That works on Oracle and SQL Server...not exactly sure about MySQL

Answer 5

How about to use REPLACE query for it? REPLACE query will replace the old one that has same primary key ( or unique key ) without an error. I think some user may want to update their votes.

Answer 6

我会更改加载方式的视图，方法是在当前用户已经为歌曲投票后不允许“投票”，或者如果您想让他们更改评分，请更新该内容，而不是在同一用户中对该用户进行插入歌曲..

Answer 7

如果已定义该表，使得（songid，userid）对于每个记录都必须是唯一的（通过将这对定义为键），则如果用户尝试对同一首歌曲进行重新投票，则插入将失败。