Getting SUM and JOIN to cooperate in a specific query - mysql

Question

I'm stuck with a certain query, it seems to be simple but it seems I might have to split it in different ways to get it to accomplish what I want to do. I tried combining, recombining and I could use some guidance.
Here's a sample table, the output and expected output.
Shop_Table
shop_id | item_type | date
1 soaps 2000-01-01
2 food 2000-01-02
3 appliances 2000-01-03
4 electronics 2000-01-10
5 furniture 2000-01-13
6 misc. 2000-01-15
Instance_Table
instanceid| shop_id | firstname | lastname | number_of_items
11 3 jane doe 2
22 2 jane doe 3
33 1 jane doe 5
44 4 jane doe 6
55 6 jane doe 1
66 5 jane doe 2
The Query
SELECT
IF
(
Shop_Table.item_type
IN
(
'soaps',
'food'
),
'indexpensives',
Shop_Table.item_type
),
COALESCE(SUM(Person.number_of_items), 0) AS item_sum
FROM Shop_Table
INNER JOIN Instance_Table AS Instance
ON Instance.shop_id = Shop_Table.shop_id
AND Instance.firstname = 'jane'
AND Instance.lastname = 'doe'
AND Shop_Table.date BETWEEN DATE( '2000-01-01' ) AND DATE( '2000-01-03' )
GROUP BY Shop_Table.item_type
Which returns:
item_type | item_sum
indexpensives | 8
appliances | 2
Output
Which is great so far, and is where I'm stuck. What I really want is the following output:
Expected Output
item_type | item_sum
inexpensives | 8
appliances | 2
electronics | 0
furniture | 0
misc. | 0
I think it's pretty straight forward what I want to do. What's making it a little tough is that it has a little bit of everything.
Grouping and combining sum results (soaps and food into one result)
Returning 0 for the rest of the item types when the date is outside the specified range.
Update:
Turned out a little more complicated than I thought, but it's what I wanted. Thank you everyone for helping out.
SELECT
IF
(
Shop_Table.item_type
IN
(
'soaps',
'food'
),
'inexpensives',
Shop_Table.item_type
),
COALESCE(SUM(Person.number_of_items), 0) AS item_sum
FROM Shop_Table
LEFT OUTER JOIN Instance_Table AS Instance
ON Instance.shop_id = Shop_Table.shop_id
AND Instance.firstname = 'jane'
AND Instance.lastname = 'doe'
AND Shop_Table.date BETWEEN DATE( '2000-01-01' ) AND DATE( '2000-01-03' )
GROUP BY IF
(
Shop_Table.item_type
IN
(
'soaps',
'food'
),
'inexpensives',
Shop_Table.item_type
)

Answer 1

Change your query to this and you should get the totals you're looking for:
SELECT
IF
(
Shop_Table.item_type
IN
(
'soaps',
'food'
),
'indexpensives',
Shop_Table.item_type
),
SUM(Person.number_of_items) AS item_sum
FROM Shop_Table
LEFT OUTER JOIN Instance_Table AS Instance
ON Instance.shop_id = Shop_Table.shop_id
AND Instance.firstname = 'jane'
AND Instance.lastname = 'doe'
AND Shop_Table.date BETWEEN DATE( '2000-01-01' ) AND DATE( '2000-01-03' )
GROUP BY Shop_Table.item_type
Explanation
An outer join will select all rows from one table and matching rows from another. So, a LEFT OUTER JOIN will select all rows from the table on the left side of the join (Shop_Table in this case) and matching rows from the table on the right side of the join (Instance_Table in this case).

Answer 2

First of all replace your inner join with a left join and then make sure you coalesce the sum with a 0, like this:
SELECT
IF
(
Shop_Table.item_type
IN
(
'soaps',
'food'
),
'indexpensives',
Shop_Table.item_type
),
COALESCE(SUM(Person.number_of_items), 0) AS item_sum
FROM Shop_Table
LEFT JOIN Instance_Table AS Instance
ON Instance.shop_id = Shop_Table.shop_id
AND Instance.firstname = 'jane'
AND Instance.lastname = 'doe'
AND Shop_Table.date BETWEEN DATE( '2000-01-01' ) AND DATE( '2000-01-03' )
GROUP BY Shop_Table.item_type
Edit:
After the requirement changes, this should be the final query:
SELECT
IF
(
Shop_Table.item_type
IN
(
'soaps',
'food'
),
'inexpensives',
Shop_Table.item_type
) as FinalType,
COALESCE(SUM(Person.number_of_items), 0) AS item_sum
FROM Shop_Table
LEFT OUTER JOIN Instance_Table AS Instance
ON Instance.shop_id = Shop_Table.shop_id
AND Instance.firstname = 'jane'
AND Instance.lastname = 'doe'
AND Shop_Table.date BETWEEN DATE( '2000-01-01' ) AND DATE( '2000-01-03' )
GROUP BY FinalType