Try with joining that may help you,
Select itm.itemId,itm.itemName,itm.itemBarcode,itm.created,itm.lastModified from tblItems itm tbl
left outer join tlbRoomItems ritm on itm.itemid = ritm.itemid
left outer join tblRoom rm on rm.roomId = ritm.roomid
rm.officeId = "12"
Your query may be slightly different. try with above query that may improve performance.