Fixing performance issues when looping through MySQL results in PHP - php

Question

I'm working on a virtual game board style game in which players get points on a certain area of the board. (Take it easy on me now as I only do this as a hobby so I might be doing this in the worst way possible)
I have 3 tables. One stores all the player information (eg. id, screenname). A second stores all the Area information (eg. id, x, y) and a third stores how many points each player has in each area (eg. id, playerid, areaid, points). In order to create a "leaderboard" I'm looping through all the players, then within that loop I'm also looping through all the areas, then finally within that second loop, I get the leader of that area and see if that matches the current player in the first loop, if so I increment a counter, then store it into an array. (See code below with some commenting)
I looked into MySQL caching, but I dont have access to a lot of the server options, as well as would like to keep as much of the results as live as possible, so caching may not be the right way to go.
My question is whether or not I'm doing this properly. Currently there is only around 10 players, and approx. 500 areas. I'm finding the below script already takes about 5-8 seconds to run. Potentially there could be millions of areas, so such a long delay in processing could be catastrophic (for the leaderboard anyway). Am I going about this the right way, and/or is there a better way to do this?
<?php
$leaders = array();
//Loop through all the players
$sql = "SELECT * FROM players";
$result = mysqli_query($con, $sql) or die(mysqli_error($con));
while ($row = mysqli_fetch_array($result)) {
//save player information into variables
$playerId = $row['id'];
$playerScreenName = $row['screenname'];
//Reset the area counter
$AreaCount = 0;
$leader = array();
//Loop through all areas
$sql2 = "SELECT * FROM areas";
$result2 = mysqli_query($con, $sql2) or die(mysqli_error($con));
while ($row2 = mysqli_fetch_array($result2)) {
$areaId = $row2['id'];
//Get the player with the most points in that area
$sql3 = "SELECT * FROM points WHERE areaid='$areaId' ORDER BY totalpoints DESC LIMIT 1";
$result3 = mysqli_query($con, $sql3) or die(mysqli_error($con));
while ($row3 = mysqli_fetch_array($result3)) {
$leaderOfArea = $row3['playerid'];
//See if the leader of the area is the same player we are looping through
if ($playerId == $leaderOfArea) {
//if it is, then increment the counter
$AreaCount++;
}
}
}
//Store the leader information into an array to be output later
$leader['screenname'] = $playerScreenName;
$leader['areacount'] = $AreaCount;
$leaders[] = $leader;
}
// sort leaders by score
usort($leaders, 'compare_areacount');
?>

Answer 1

There is overhead of opening database connections, and when you do it in a loop, you exacerbate the problem (and then when you add a loop inside of that, you make it that much worse). Instead, restructure it as one query using a Join or Subquery.
This post has some specifics.

Answer 2

I think this code can help. You might have to change the sql queries with appropriate column names you need.
<?php
$leaders = array();
//Loop through all the players
$sql = "SELECT * FROM players";
$result = mysqli_query($con, $sql) or die(mysqli_error($con));
$players = array();
while ($row = mysqli_fetch_array($result)) {
//save player information into variables
$players[$row['id']] = array($row['screenname'], 0);
// number 0 will be the count of how many times this player is the leader
}
$sql = "SELECT Area.id, Area.name, (SELECT Pts.playerid FROM `points`"
. " AS Pts WHERE Pts.areaid=Area.id ORDER BY totalpoints"
. " DESC LIMIT 1) AS `leader_id` FROM `areas` AS Area";
$result = mysqli_query($con, $sql) or die(mysqli_error($con));
$areas = array();
while ($row = mysqli_fetch_row($result)) {
$areas[$row[0]] = $row;
// $row[2] will contain leader_id
// index 1 corresponds to the second element in player values array
$players[$row[2]][1]++;
}
// now if you want to print:
foreach ($areas as $area_id => $area) {
echo "Area id: " . $area_id . ", name: " . $area[1] . ", leader_id: " . $area[2] . "<br /><br />";
}
foreach ($players as $player_id => $player) {
echo "Player id: " . $player_id . ", name: " . $player[0] . ", No of areas this player is a leader of: " . $player[1] . "<br /><br />";
}