join values of 2 tables and display data - mysql

Question

I have 2 tables, cart and product. User can choose product from product table and then its details are added to cart table. i wanted a listing that combines the values of these two tables and gives a result
prodsize
id catid catname productid prodsize cost prodname
1 2 CN1 13 small 130 P1
2 2 CN1 13 large 200 P1
3 2 CN1 14 small 50 P2
4 2 CN1 14 medium 90 P2
5 2 CN1 14 large 110 P2
6 2 CN1 12 small 70 P3
7 2 CN1 12 medium 110 P3
8 2 CN1 13 medium 200 P1
cart
id catid catname userid productid prodname prodsize prodcost quantity status
1 2 CN1 1 13 P1 small 130 2 add_to_cart
1 2 CN1 1 13 P1 large 200 2 order_placed
when i run the following query combined value from both the table but here i want that only those products quantity should get displayed whose status is add_to_cart, if the status is order_placed then the quantity should be 0, can anyone plz tell how can i modify the below code to get desired result
$sql= "SELECT p.catid, p.catname, p.productid, p.prodimg, GROUP_CONCAT(p.prodsize ORDER BY p.id ASC) as size, GROUP_CONCAT(p.cost ORDER BY p.id ASC) as cost, p.prodname,
GROUP_CONCAT(c.prodsize,'-',c.quantity) as cart_details, GROUP_CONCAT(DISTINCT(c.userid)) as user_id
FROM productsize p
LEFT JOIN cart c ON(c.productid = p.productid AND c.userid = '$userid' AND p.prodsize = c.prodsize)
WHERE p.catid ='$catid'
GROUP BY p.productid
ORDER BY user_id DESC, p.productid ASC";

Answer 1

I'm a little confused about your two conditions ("ordered" items shouldn't get displayed, and should have quantity 0). If they have a quantity 0, they need to be displayed, right? Anyhow, here's my fix considering you may want to do the latter (get "ordered" items in your query result with a quantity of 0)
You need to group by productid AND status
You need a quantity as an aggregate SUM that takes the status into account
Cleaned up some other things too (e.g. quoted indexes, which will harm performance):
$sql = "SELECT p.catid, p.catname, p.productid, p.prodimg,
GROUP_CONCAT(p.prodsize ORDER BY p.id ASC) as size,
GROUP_CONCAT(p.cost ORDER BY p.id ASC) as cost, p.prodname,
GROUP_CONCAT(c.prodsize,'-',c.quantity) as cart_details,
GROUP_CONCAT(DISTINCT(c.userid)) as user_id,
SUM(CASE WHEN c.status = 'add_to_cart' THEN quantity ELSE 0 END) AS quantity
FROM productsize p
LEFT JOIN cart c ON(c.productid = p.productid
AND p.prodsize = c.prodsize)
AND c.userid = {$userid}
WHERE p.catid = {$catid}
GROUP BY p.productid, c.status
ORDER BY p.productid ASC";

Answer 2

You've already got the where clause. Try adding
AND status = 'add_to_cart'
just after the existing where clause.

Answer 3

Change c.quantity to:
case c.status
when 'add_to_cart' then c.quantity
else 0
end
Since you used a left join, the zero result will also apply when that product didn't appear in the user's cart.
And note that the sort on userid may be effectively ordering the cart and non-cart products but that is the wrong way to express that intention. It's also best to aggregate the non-group columns everywhere.