SQL Query Running super slow - php

Question

The code I'm running below runs really quite fine, until I try and fetch the Quantity in the last loop. I think it could probably be optimized with JOINs etc but I'm not overly familiar with the best approach for this when grabbing DISTINCT elements?
$sql_dept = "SELECT DISTINCT `department_guid` FROM `bom_material_copy` WHERE `complex_guid`='9EB75BE9-26E2-AEBB-1B52-37DBC2FB89EA' ORDER BY `id` DESC LIMIT 0,5";
foreach($pdo->query($sql_dept) AS $dept)
{
$sql_mat = "SELECT DISTINCT `material_bct_number` FROM `bom_material_copy` WHERE
`complex_guid`='9EB75BE9-26E2-AEBB-1B52-37DBC2FB89EA' AND
`department_guid`='".$dept['department_guid']."'
ORDER BY `id` DESC LIMIT 0,5";
foreach($pdo->query($sql_mat) AS $mat)
{
echo "<tr>";
echo "<td>".$mat['material_bct_number']."</td>";
// Grab Units Again
$u = 0;
$sql_unit = "SELECT DISTINCT `unit_number` FROM `bom_projects_units` WHERE `complex_guid`='9EB75BE9-26E2-AEBB-1B52-37DBC2FB89EA'";
foreach($pdo->query($sql_unit) AS $unit)
{
$sql_quan = "SELECT `quantity` FROM `bom_material_copy`
WHERE
`material_bct_number`='".$mat['material_bct_number']."' AND
`unit_number`='".$unit['unit_number']."' AND
`department_guid`='".$dept['department_guid']."'";
foreach($pdo->query($sql_quan) AS $quan)
{
echo "<td>".$quan['quantity']."</td>";
}
}
echo "</tr>";
}
}

Answer 1

Try using EXPLAIN in your queries.
When you do that, you know the relationship between tables that you're using in the query.
Next step, use INDEX in the fields that you're making match, and execute again you're queries.