Sum gave me wrong amount - mysql

Question

This is my table:
(Stuff Table)
id_stuff | prody_type | semester | amount_prod
1 090 1 10
2 210 2 35
(Amount Table)
id_amount | prod_type | semester | amount_stuff
1 090 1 12
2 210 2 15
(Product Table)
id_prod | type_prod | prod_number
1 090 010
2 210 020
And here's my code from my model file:
$this->db->select("sum(amount_stuff) as 'tot_amount_stuff'")
->from('stuff')
->join('product','prod_type=type_prod')
->join('amount','stuff.prod_type=amount.prod_type')
->WHERE('amount.semester', 2);
Then code to show in my page:
for each.......
<td><?php echo $row->tot_amount_stuf; ?></td>
And in my html page shows: 50 but that's wrong, the correct amount is 15
It's seems the 'sum' taking all from the tables 'Stuff' and 'Amount' on semester 2, but I only need to take/show the sum of amount_stuff from the Amount Table not from both tables.
I'm little bit confused here...Hope anyone can help me.
Best Regards,

Answer 1

Try
SELECT sum(amount_stuff) as 'tot_amount_stuff'
FROM Amount
JOIN Stuff ON (Stuff.prody_type = Amount.prody_type)
JOIN product ON (Amount.prody_type = Product.type_prod)
WHERE amount.semester = 2;

Answer 2

Make sure you're referencing tables and columns in your select
$this->db->select("sum(amount.amount_stuff) as 'tot_amount_stuff'")
->from('stuff')
->join('product','prod_type=type_prod')
->join('amount','stuff.prod_type=amount.prod_type')
->WHERE('amount.semester', 2);
You'll want your SQL to render like so:
SELECT sum(amount.amount_stuff) as 'tot_amount_stuff'
FROM stuff
JOIN product ON (amount.prod_type = product.type_prod)
JOIN amount ON (stuff.prod_type = amount.prod_type)
WHERE amount.semester = '2'
You could also use aliases to shorten this process
SELECT sum(a.amount_stuff) as 'tot_amount_stuff'
FROM stuff s
JOIN product p ON (a.prod_type = p.type_prod)
JOIN amount a ON (s.prod_type = a.prod_type)
WHERE a.semester = '2'