MySQL JOIN "WHEN"- is such a thing possible? - mysql

Question

Background:
I'm running a query which gets the total widgets, sprockets, and gizmos ordered by the customer during the last month
I also have a Total Orders column which runs a subquery counting every order in that month
The reason I do Total Orders in a subquery, instead of just adding the three columns, is because I also have a further two Total Orders columns for the two previous months
I have an orders table that stores financial records, and separate tables for actual product details for widgets, sprockets, and gizmos
The problem:
Occasionally, a widget might be 'half-deleted' (don't ask!) - where it has an orders record, but not a corresponding widgets record
I do not want to count this in the Total Widgets column - which is easy enough, as I just do a JOIN
However, I also do not want to count this in the Total Orders column...
My current query looks like this for Total Widgets:
SELECT
COUNT(orders.id)
FROM orders
JOIN widgets ON widgets.id = orders.item_id
WHERE orders.product_id = 1 -- Product ID 1 is a Widget
AND orders.date BETWEEN "2014-09-01 00:00:00" AND "2014-09-30 23:59:59"
So this will get all 'accurate' widgets, with an intact widget table record.
Here is my current query for Total Orders:
SELECT
COUNT(orders.id) AS count
FROM orders
JOIN widgets ON widgets.id = orders.item_id AND orders.product_id = 1
WHERE orders.date BETWEEN "2014-09-01 00:00:00" AND "2014-09-30 23:59:59"
So my thinking is that the above query should only JOIN when the order has a product_id of 1. However, it JOINs in every case. Which means if we've ordered 10 widgets (2 of which have been half-deleted), 5 sprockets, and 5 gizmos, rather than showing 18 orders, it only shows 8. Changing to a LEFT JOIN shows 20, which is still wrong, it should be 18.
Hopefully the above makes sense - thanks in advance.

Answer 1

okay so... the straight join doesn't work because you filter for orders.product_id = 1 and so it won't count sprockets or gizmos. just widgets. and if you have 2 half deleted, but 10 on record... 8 is the correct answer. the left outer join lets it join on rows where product_id <> 1, too, so all rows from orders (which contains order records on widgets, sprockets, gizmos - half deleted or not). but this also allows those 2 half deleted rows.
why don't you just do a straight join? if you want the answer 18, just join orders and products and count it. do not filter for widgets only. do not do a left outer if you don't want half deleted.
create table product (
product_id int,
product_name varchar(20)
);
insert into product values (1,'widget');
insert into product values (2,'sprocket');
insert into product values (3,'gizmo');
create table `order` (
order_id int,
widget_id int
);
insert into `order` values (1,1);
insert into `order` values (1,2);
insert into `order` values (1,3);
insert into `order` values (1,4); -- half deleted
insert into `order` values (2,1);
insert into `order` values (2,2);
insert into `order` values (3,3);
insert into `order` values (3,4); -- half deleted
insert into `order` values (4,4); -- half deleted
select o.order_id, count(*)
from `order` as o
join product as p
on o.widget_id=p.product_id
group by o.order_id
+----------+----------+
| order_id | count(*) |
+----------+----------+
| 1 | 3 |
| 2 | 2 |
| 3 | 1 |
+----------+----------+
3 rows in set (0.00 sec)
mysql> select o.order_id,
-> p.product_name,
-> (case when p.product_id is null
-> then 'half_deleted' else '' end) as half_deleted
-> from `order` as o
-> left outer join product as p
-> on o.widget_id=p.product_id
-> order by order_id ;
+----------+--------------+--------------+
| order_id | product_name | half_deleted |
+----------+--------------+--------------+
| 1 | gizmo | |
| 1 | NULL | half_deleted |
| 1 | widget | |
| 1 | sprocket | |
| 2 | widget | |
| 2 | sprocket | |
| 3 | gizmo | |
| 3 | NULL | half_deleted |
| 4 | NULL | half_deleted |
+----------+--------------+--------------+