嵌套的sql查询，按组分组，并为每个组返回有限的行

Question

Trying to get the right query to look through my table which looks like this:

id    fromName    toName    messages
1     user1       me        .......
2     user2       me        .......
3     user1       me        .......
4     user1       me        .......
5     user3       me        .......
6     user2       me        .......

What I want to do is get an array which has all the messages from user1 in an array, user2 in an array, user3...etc. I had a query that did that:

$query = mysqli_query($link, "SELECT * FROM messages WHERE toName='$to' OR fromName='$to' order by id desc");
$messages = array();
while ($row = mysqli_fetch_assoc($query)) {
    $messages[$row['fromName']][] = $row;
}

Now I want to put a limit on how many rows it gets back...for example a limit of 2 rows from all users. Do I need a nested query using "group by"? This query isn't working:

$query = mysqli_query($link, "SELECT * FROM messages WHERE toName='$to' OR fromName='$to' group by fromName order by id desc limit 2");
//or this one
    $query = mysqli_query($link, "SELECT * FROM messages WHERE toName='$to' OR fromName='$to' AND id IN(SELECT * FROM messages group by fromName)");
$messages = array();
while ($row = mysqli_fetch_assoc($query)) {
    $messages[$row['fromName']][] = $row;
}

Hope this isn't too difficult to solve....Thanks in advance!

Answer 1

Why not use your original query and run a count on the array before assigning another.

if(count($messages[$row['fromName']]) < 2)
    $messages[$row['fromName']][] = $row;
}

Answer 2

You should really use GROUP BY in your query to get results. You will not need to iterate through the results and formulate an array.

Updated code:

$query = mysqli_query($link, "SELECT * FROM messages WHERE toName='$to' OR fromName='$to' GROUP BY fromName");
$senderMessages = mysqli_fetch_assoc($query);