Subquery in PHP

Question

Let's put an easy example with two tables:

USERS (Id, Name, City)
PLAYERS (Id_Player, Number, Team)

And I have to do a query with a subselect in a loop, where the subselect is always the same, so I would like to divide it into two queries and put the subselect outside the loop.

I explain. What works but it is not optimize:

for($i=0;$i<something;$i++)
{
    $res2=mysql_query("SELECT Team from PLAYERS WHERE Number=$i 
        AND Id_Player IN (SELECT Id FROM USERS WHERE City='London')");
}

What I would like to do but it doesn't work:

$res1=mysql_query("SELECT Id from USERS where City='London'");
for($i=0;$i<something;$i++)
{
    $res2=mysql_query("SELECT Team from PLAYERS WHERE Number=$i 
        AND Id_Player IN **$res1**");
}

Thanks!

Answer 1

Something like this should work.

<?
$sql = "SELECT Team from PLAYERS 
    JOIN USERS on (Id_player=Id)
    WHERE Number BETWEEN $minID AND $maxID
    AND City='London'
    GROUP BY Team";

$results=mysql_query($sql) or die(mysql_error());


// $results contain all the teams from London
// Use like normal..

echo "<ul>\n";

while($team = mysql_fetch_array($results)){
    echo "\t<li>{$team['Team']}</li>\n";
}

echo "</ul>";

Answer 2

Placing SQL quires in loops can be very slow and take up a lot of resources, have a look at using JOIN in you SQL. It's not that difficult and once you've got the hang of it you can write some really fast powerful SQL.

Here is a good tutorial worth having a look at about the different types of JOINs:
http://www.keithjbrown.co.uk/vworks/mysql/mysql_p5.php

Answer 3

SELECT PLAYERS.*, USERS.City FROM PLAYERS, USERS WHERE USERS.City='London' AND PLAYERS.Number = $i

Not the best way to do it; maybe a LEFT JOIN, but it should work. Might have the syntax wrong though.

James

EDIT

WARNING: This is not the most ideal solution. Please give me a more specific query and I can sort out a join query for you.

Taking your comment into account, let's take a look at another example. This will use PHP to make a list we can use with the MySQL IN keyword.

First, make your query:

$res1 = mysql_query("SELECT Id from USERS where City='London'");

Then, loop through your query and put each Id field one after another in a comma seperated list:

$player_ids = "";

while($row = mysql_fetch_array($res1))
{
    $player_ids .= $row['Id'] . ",";
}

$player_ids = rtrim($player_ids, ",");

You should now have a list of IDs like this:

12, 14, 6, 4, 3, 15, ...

Now to put it into your second query:

for($i = 0; $i<something; $i++) 
{      
    $res2 = mysql_query("SELECT Team from PLAYERS WHERE Number=$i          
    AND Id_Player IN $player_ids"); 
}

The example given here can be improved for it's specific purpose, however I'm trying to keep it as open as possible.

If you want to make a list of strings, not IDs or other numbers, modify the first while loop, replacing the line inside it with

$player_ids .= "'" . $row['Id'] . "',";

If you could give me your actual query you use, I can come up with something better; as I said above, this is a more generic way of doing things, not necessarily the best.

Answer 4

Running query in a loop is not a great idea. Much better would be to get whole table, and then iterate through table in loop.

So query would be something like that:

"SELECT Team from PLAYERS WHERE Number BETWEEN($id, $something) 
        AND Id_Player IN (SELECT Id FROM USERS WHERE City='London')"

Answer 5

$res1=mysql_query("SELECT Id from USERS where City='London'");
for($i=0;$i<something;$i++)
{
    $res2=mysql_query("SELECT Team from PLAYERS WHERE Number=$i 
        AND Id_Player IN **$res1**");
}

Would work, but mysql_query() returns a RESULT HANDLE. It does not return the id value. Any select query, no matter how many, or few, rows it returns, returns a result statement, not a value. You first have to fetch the row using one of the mysql_fetch...() calls, which returns that row, from which you can then extract the id value. so...

$stmt = mysql_query("select ID ...");
if ($stmt === FALSE) {
    die(msyql_error());
}
if ($stmt->num_rows > 0) {
    $ids = array();
    while($row = mysql_fetch_assoc($stmt)) {
        $ids[] = $row['id']
    }
    $ids = implode(',', $ids)
    $stmt = mysql_query("select TEAM from ... where Id_player IN ($ids)");
    .... more fetching/processing here ...
}