MySQL slow query

phpMyAdmin 2.10.0.2 / MySQL 5.0.45

I have written an inventory program. My database contains these relevant tables: Inventory and Client_Inv. The inventory table contains all items. The Client_Inv contains the information necessary to determine which items belong to which clients - as well as the full item name, location, and limits. The inventory table contains a row for each item - so an item with a quantity of 500 will have 500 entries into the table to show when items were added, removed, or on order.

My query is designed to show all relevant information (location, item name, quantity on hand, and what is currently on order)  to the end user on a PHP page.

On the PHP page there is a drop down menu where the user selects which client's inventory to view and it then runs the query attached.

The query currently takes 13 seconds to run and I'm trying to determine how to possibly cut that down. On clients with inventory for each item, the time is considerably lower - however, if a client has an item associated to them but no available inventory - the query slows down. I've tried just running the first part of the query and it is incredibly slow without the UNION statement.

I have attached the explain details. Any information or direction on how to possibly beat my current 13 second execution time would be greatly appreciated. Thanks!

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(SELECT Client_Inv.ItemID, Client_Inv.ItemName, Client_Inv.Location, count(*) AS count, 
SUM(IF(Client_Inv.ItemID = Inventory.ItemID AND OnOrder= '1', 1, 0)) AS Acount FROM Client_Inv
 LEFT JOIN Inventory ON Client_Inv.ItemID = Inventory.ItemID WHERE Client_Inv.Client_ID = '$q' AND 
Client_Inv.Discod = '0000-00-00' AND Inventory.RemDate = '0000-00-00' AND Inventory.Discod = '0' 
GROUP BY ItemID)
 
 UNION 
 
(SELECT Client_Inv.ItemID, Client_Inv.ItemName, Client_Inv.Location, count(NULL) AS num,
 SUM(IF(Client_Inv.ItemID = Inventory.ItemID AND OnOrder= '0', 0, 0)) AS numb FROM Client_Inv, Inventory WHERE
 Client_Inv.Client_ID = '$q' AND Client_Inv.Discod = '0000-00-00' AND Client_Inv.ItemID NOT IN 
(SELECT DISTINCT ItemID FROM Inventory WHERE Inventory.RemDate = '0000-00-00') GROUP BY ItemID) Order By 
ItemName