Question

I cannot seem to get this MySQL query right. My table contains yearly inventory data for retail stores. Here’s the table schema:

CREATE TABLE IF NOT EXISTS `inventory_data` (
inventory_id int unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,
store_id smallint unsigned NOT NULL,
inventory_year smallint unsigned NOT NULL,
shortage_dollars decimal(10,2) unsigned NOT NULL
)
engine=INNODB;

Every store is assigned to a district which in this table (some non-relevant fields removed):

CREATE TABLE IF NOT EXISTS `stores` (
store_id smallint unsigned NOT NULL AUTO_INCREMENT PRIMARY KEY,
district_id smallint unsigned not null
)
engine=INNODB;

I want to be able to retrieve the shortage dollar amounts for two given years for all the stores within a given district. Inventory data for each store is only added to the inventory_data table when the inventory is completed, so not all stores within a district will all be represented all the time.

This query works to return inventory data for all stores within a given district for a given year (ex: stores in district 1 for 2012):

SELECT stores.store_id, inventory_data.shortage_dollars 
FROM stores 
LEFT JOIN inventory_data ON (stores.store_id = inventory_data.store_id) 
AND inventory_data.inventory_year = 2012 
WHERE stores.district_id = 1

But, I need to be able to get data for stores within a district for two years, such that the data looks something close to this:

store_id | yr2011 | yr2012

Answer 1

For the specific result format that you need, you may try the following query:

SELECT `s`.`store_id`, `i`.`shortage_dollars` AS `yr2011`, `i1`.`shortage_dollars` AS `yr2012`
FROM `stores` `s`
LEFT JOIN `inventory_data` `i` ON `s`.`store_id` = `i`.`store_id`
    AND `i`.`inventory_year` = 2011
LEFT JOIN `inventory_data` `i1` ON `s`.`store_id` = `i1`.`store_id`
    AND `i1`.`inventory_year` = 2012
WHERE `s`.`district_id` = 1

Alternatively, you may as well try the next simpler query.

SELECT `s`.`store_id`, `i`.`inventory_year`, `i`.`shortage_dollars`
FROM `stores` `s`
LEFT JOIN `inventory_data` `i` ON `s`.`store_id` = `i`.`store_id`
WHERE `s`.`district_id` = 1
AND `i`.`inventory_year` IN (2011, 2012)
ORDER BY `s`.`store_id`, `i`.`inventory_year`

Hope it helps!