MYSQL inventory slots (add item to next slot free) - mysql

Question

$slt = mysql_query("select Slot, ItemId, UserId, max(Slot) Slot
from useritems
group by UserId");
while ($sloot = mysql_fetch_assoc($slt))
{
Echo "<br>Items with biggest slots are: " . $sloot['ItemId'] . " from user " . $sloot['UserId']. "- in slot-". $sloot['Slot'];
}
This is the table
Idi Quantity ItemId UserId Slot ExpirationDate
Outputs the smallest Slots...Why?
1.I want to show me the biggest inventory slot from each user, so when i add a new item to his inventory i can add on next slot..From example user Paul has 5 items that ocupies slots 1,2,3,4,5 and the next item will be on slot 6.
2.When a user moves his items on slots 1,2,4,5,6 the next item added will be on slot 3
I did a lot of search but i can't find out myself:) PS:The game wich im making its just for fun..but maybe someday will be a great game:) (dreams,dreams :)) )
EDIT:
SQLFIDDLE is very good thank you:) it's exactly what i need to learn some SQL
Table useritems
useritems Table IMAGE
items Table
My Echo shows me that:
Id = 1 and it should be 3; user=4(good); slot=4(good)
Id = 1 and it should be 2; user=5(good); slot=2(good)

Answer 1

Maybe something like:
select userid, itemid, max(slot) from useritems where itemid is not null and quantity>0
group by userid, itemid
It'd be easier to help you if you share your table script with some data.
You can use: http://sqlfiddle.com/
EDIT:
What about this? Itemid=0 means the slot is free? So min(slot) will be the first free slot by user.
select userid, min(slot) from useritems where itemid=0 group by userid

Answer 2

I found it!:) i just forgot to select data where UserId=my_id but it shows me the corect output only if the respective user has more than 3 items...
$slt = mysql_query("SELECT * FROM useritems WHERE Slot=(select max(Slot) from useritems) and UserId='$id'");
and $id = $_SESSION['id'];
$slt = mysql_query("
SELECT * FROM `useritems`
WHERE UserId='$id' AND Slot=(select max(Slot)
from useritems where UserId='$id')
");`
EDIT2:I found the best way with all i was searching but i don`t knwo how to use WHERE clause userId='$id'
$slt2 = mysql_query("select l.Slot + 1 as start
from useritems as l
left outer join useritems as r on l.Slot + 1 = r.Slot
where r.Slot is null and l.UserId=4;
") or die(mysql_error()); `
With this query the item should be placed on Slot 2(wich is missing) but it`s dysplays Slot 6 (wich is the highest for the user with UserId=4)
Finally this is the last edit
$slt2 = mysql_query("SELECT Slot + 1 as start
FROM useritems mo
WHERE NOT EXISTS
(
SELECT NULL
FROM useritems mi
WHERE mi.Slot = mo.Slot + 1 AND mi.UserId = '$id'
)ORDER BY Slot LIMIT 1
") or die(mysql_error()); `
This is what i was searching.:X