I am trying to write a query to determine how much of my inventory is committed at a given time, i.e. current, next month, etc.
A simplified example:
I have an inventory table of items. I have an offer table that specifies the customer, when the offer starts, and when the offer expires. I have a third table that associates the two.
create table inventory
(id int not null auto_increment , name varchar(32) not null, primary key(id));
create table offer
(id int not null auto_increment , customer_name varchar(32) not null, starts_at datetime not null, expires_at datetime, primary key (id));
create table items
(id int not null auto_increment, inventory_id int not null, offer_id int not null, primary key (id),
CONSTRAINT fk_item__offer FOREIGN KEY (offer_id) REFERENCES offer(id),
CONSTRAINT fk_item__inventory FOREIGN KEY (inventory_id) REFERENCES inventory(id));
create some inventory
insert into inventory(name)
values ('item 1'), ('item 2'),('item 3');
create two offers for this month
insert into offer(customer_name, starts_at)
values ('customer 1', DATE_FORMAT(NOW(), '%Y-%m-01')), ('customer 2', DATE_FORMAT(NOW(), '%Y-%m-01'));
and one for next month
insert into offer(customer_name, starts_at)
values ('customer 3', DATE_FORMAT(DATE_ADD(CURDATE(), INTERVAL 1 MONTH), '%Y-%m-01'));
Now add some items to each offer
insert into items(inventory_id, offer_id)
values (1,1), (2,1), (2,2), (3,3);
What I want is a query that will show me all the inventory and the count of the committed inventory for this month. Inventory would be considered committed if the starts_at is less than or equal to now, and the offer has not expired (expires_at is null or expires_at is in the future)
The results I would expect would look like this:
+----+--------+---------------------+
| id | name | committed_inventory |
+----+--------+---------------------+
| 1 | item 1 | 1 |
| 2 | item 2 | 2 |
| 3 | item 3 | 0 |
+----+--------+---------------------+
3 rows in set (0.00 sec)
The query that I felt should work is:
SELECT inventory.id
, inventory.name
, count(items.id) as committed_inventory
FROM inventory
LEFT JOIN items
ON items.inventory_id = inventory.id
LEFT JOIN offer
ON offer.id = items.offer_id
WHERE (offer.starts_at IS NULL OR offer.starts_at <= NOW())
AND (offer.expires_at IS NULL OR offer.expires_at > NOW())
GROUP BY inventory.id, inventory.name;
However, the results from this query does not include the third item. What I get is this:
+----+--------+---------------------+
| id | name | committed_inventory |
+----+--------+---------------------+
| 1 | item 1 | 1 |
| 2 | item 2 | 2 |
+----+--------+---------------------+
2 rows in set (0.00 sec)
I cannot figure out how to get the third inventory item to show. Since inventory is the driving table in the outer joins, I thought that it should always show.