Multiple date range stock overbooked query

Question

I have been struggling a bit with a query and I'm not sure if its possible in pure sql or needs to be backed by php.

Basically I want to generate an order report listing the products that have been overbooked in the system.

Table structure:

orders

id int
bookingStart DATETIME
bookingEnd DATETIME

order_lines

id int
qty int
product_id int
booking_id int

products

id int
stock int

An order can contain multiple lines with the same product. So there needs to be a SUM for each product on an order. Does anyone have any suggestions for the best way to accomplish this?

I have tried looping in a specific time interval with a subquery, but it does not take into account the overlap between different bookings and im kinda stuck here, if it can be done with a single query:

SELECT  (SELECT SUM(lin2.qty) FROM booking_lines lin2,orders b2
                       WHERE
                         lin2.product_id=e.id    AND
                       b2.id=lin2.booking_id  AND                           (
                       (b2.bookingStart BETWEEN b1.bookingStart AND b1.bookingEnd) OR
                       (b2.bookingEnd BETWEEN b1.bookingStart AND b1.bookingEnd) OR
                       (b2.bookingStart < b1.bookingStart AND b2.bookingEnd >     b1.bookingEnd)                           
                       )    as booked SUM(lin1.qty) 
                 FROM orders b1
                 LEFT JOIN order_lines lin1 ON b1.id=lin1.booking_id
                 LEFT JOIN products e ON e.id=lin1.product_id

                (
                    (b1.bookingStart BETWEEN '$s' AND '$e') OR
                    (b1.bookingEnd BETWEEN '$s' AND '$e') OR
                    (b1.bookingStart < '$s' AND b1.bookingEnd > '$e') 
        )
         GROUP BY b1.id,lin1.product_id

The problem If we have product x with a stock of 10.

Order 1 has booked 3 product x Order 2 has booked 5 product x Order 3 has booked 8 product x

The lines below indicate the booked timespan

______Order 1 x:3____
       ___Order 2 x:5_____
__________Order 3 x:8______
                                            __order 4 x:2_

So its only when the 3 orders overlap that product x is actually overbooked, what i cant figure out is how to create a query capable of detecting that.

Answer 1

Table structure

CREATE TABLE products (
    product_id INT AUTO_INCREMENT PRIMARY KEY,
    name       VARCHAR(255),
    stock      INT
) ENGINE=InnoDB;

CREATE TABLE orders (
    order_id     INT AUTO_INCREMENT PRIMARY KEY,
    bookingStart DATETIME,
    bookingEnd   DATETIME
) ENGINE=InnoDB;


CREATE TABLE order_lines (
    line_id       INT AUTO_INCREMENT PRIMARY KEY,
    order_id      INT,
    product_id    INT,
    qty           INT NOT NULL DEFAULT 0,
    CONSTRAINT FOREIGN KEY(order_id)   REFERENCES orders(order_id) ON DELETE RESTRICT,
    CONSTRAINT FOREIGN KEY(product_id) REFERENCES products(product_id) ON DELETE RESTRICT
) ENGINE=InnoDB;


INSERT INTO products(name, stock) VALUES('Product 1', 8);
INSERT INTO products(name, stock) VALUES('Product 2', 14);
INSERT INTO products(name, stock) VALUES('Product 3', 25);

INSERT INTO orders(bookingStart,bookingEnd) VALUES(NOW(), (NOW() + INTERVAL 2 HOUR));
INSERT INTO orders(bookingStart,bookingEnd) VALUES(NOW(), (NOW() + INTERVAL 3 HOUR));
INSERT INTO orders(bookingStart,bookingEnd) VALUES(NOW(), (NOW() + INTERVAL 4 HOUR));

INSERT INTO order_lines(order_id, product_id, qty) VALUES(1, 1, 2);
INSERT INTO order_lines(order_id, product_id, qty) VALUES(1, 1, 8);
INSERT INTO order_lines(order_id, product_id, qty) VALUES(2, 2, 5);
INSERT INTO order_lines(order_id, product_id, qty) VALUES(2, 2, 10);
INSERT INTO order_lines(order_id, product_id, qty) VALUES(3, 3, 2);
INSERT INTO order_lines(order_id, product_id, qty) VALUES(3, 3, 8);
INSERT INTO order_lines(order_id, product_id, qty) VALUES(3, 3, 10);

To get the overbooked products:

SELECT
  products.*,
  SUM(order_lines.qty) as sum_qty
FROM products
LEFT JOIN order_lines ON order_lines.product_id = products.product_id
LEFT JOIN orders ON orders.order_id = order_lines.order_id
WHERE orders.bookingStart >= '2013-06-12' 
AND orders.bookingEnd <= '2013-06-14'
GROUP BY order_lines.order_id, order_lines.product_id
HAVING sum_qty > products.stock

Will return Product 1 and Product 2, because the sum of qty in order_lines is bigger than products.stock (which is probably the available quantity of the product?).

SqlFiddle