MySQL query to show combinations of output - mysql

Question

I have to write a mysql query to find total sales on particular days. I am able to find it, but unable to show combined results.
The table looks like:
Date, Order_id, Product_id, Quantity
'01-JUL-11',O1,P1,5
'01-JUL-11',O2,P2,2
'01-JUL-11',O3,P3,10
'01-JUL-11',O4,P1,10
'02-JUL-11',O5,P3,5
'02-JUL-11',O6,P4,6
'02-JUL-11',O7,P1,2
'02-JUL-11',O8,P5,1
'02-JUL-11',O9,P6,2
'02-JUL-11',O10,P2,4
Desired Output:
Product_id Total Sales on Day '02-JUL-11' Total Sales on Day '02-JUL-11'
P1 15 2
P2 2 4
P3 10 5
P4 0 6
P5 0 1
P6 0 2
What I have tried is:
Select distinct product_id
from orders;
Output:
P1
P2
P3
P4
P5
P6
Select product_id, sum(quantity) from orders
where order_day = '11-07-01'
group by product_id
OUTPUT:
P1 15
P2 2
P3 10
Select product_id, sum(quantity) from orders
where order_day = '11-07-02'
group by product_id
OUTPUT:
P1 2
P2 4
P3 5
P4 6
P5 1
P6 2
This gives me the desired result but I want to somehow combine the columns.
Query that worked for me: (However puts null for 0)
Select X.product_id, X.s, Y.t from
(SELECT A.product_id as product_id, B.s as s FROM
(Select distinct product_id
from orders) A
LEFT JOIN
(Select product_id, sum(quantity) as s from orders
where order_day = '11-07-01'
group by product_id) B
ON A.product_id = B.product_id) X
Left join
(Select product_id, sum(quantity) as t from orders
where order_day = '11-07-02'
group by product_id) Y
on X.product_id = Y.product_id;

Answer 1

If you need only two days summarized in your report, then a simple (non dynamic) pivot query should do the trick:
SELECT product_id,
SUM(CASE WHEN order_day = '11-07-01' THEN quantity ELSE 0 END) AS `Total Sales on Day '01-JUL-11'`,
SUM(CASE WHEN order_day = '11-07-02' THEN quantity ELSE 0 END) AS `Total Sales on Day '02-JUL-11'`
FROM orders
WHERE order_day = '11-07-01' OR order_day = '11-07-02'
GROUP BY product_id

Answer 2

Conditional aggregation is the way to go. But you should learn to use proper date formats. The ISO/ANSI standard is YYYY-MM-DD or YYYYMMDD:
Select product_id,
sum(case when order_day = '2011-07-01' then quantity else 0 end) as q_20110701,
sum(case when order_day = '2011-07-02' then quantity else 0 end) as q_20110702
from orders o
where order_day in ('2011-07-01', '2011-07-02')
group by product_id;
An alternative, if you can live with separate rows for each day, is:
Select product_id, order_day, sum(quantity)
from orders
where order_day in ('2011-07-01', '2011-07-02')
group by product_id, order_day,