MySQL Question
MySQL query to calculate latest average prices using data from multiple tables
I am working on a PHP and MySQL based system to organise products and expenses for a restaurant.
I have data organised in to four tables.
Items table
id | name
1 | Beer
2 | Vodka
Products table
id | item_id | name
1 | 1 | Budweiser
2 | 1 | Sam Adams
3 | 2 | Smirnoff
4 | 2 | Grey Goose
Supplier table
id | name
1 | Supplier 1
2 | Supplier 2
Expenses table
id | product_id | cost | quantity | supplier | date
1 | 1 | 2.99 | 1 | 1 | 2017-09-05
2 | 1 | 3.00 | 2 | 2 | 2017-09-10
3 | 1 | 2.50 | 1 | 1 | 2017-09-20
4 | 1 | 3.98 | 2 | 1 | 2017-09-22
5 | 1 | 4.00 | 1 | 2 | 2017-09-25
6 | 1 | 8.00 | 2 | 2 | 2017-09-27
I would like to write a MYSQL Query that can figure out the cheapest supplier of a specific product based on the average cost per item (cost/quantity) of the latest 3 entires in the expenses table (based on date).
Here's what I want to compute:
Supplier 1 last 3 entries - costs per unit of: 2.99, 2.50 and 1.99. Average = 2.49
Supplier 2 last 3 entries - costs per unit of: 1.50, 4.00 and 4.00. Average = 3.16
So the SQL should return that Supplier 1 is the cheapest option for Product 1 (Budweiser).
So far I have attempted this, but I am a bit lost and confused:
select * from products
INNER JOIN expenses
ON products.id = expenses.product
AND products.item = '1'
ORDER BY (expenses.cost/expenses.quantity)
LIMIT 3;
The output of this query is which is a long way from what I'm trying to figure out :(:
id | item_id | name | id | product_id | cost | quantity | supplier | date
1 | 1 |Budweiser| 2 | 1 | 3.00 | 2 | 2 | 2017-09-10
1 | 1 |Budweiser| 4 | 1 | 3.98 | 2 | 1 | 2017-09-22
1 | 1 |Budweiser| 3 | 1 | 2.50 | 1 | 1 | 2017-09-20
The output I am looking for based on the sample data would be:
cheapest_supplier
1
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Answer Source
I would like to write a MYSQL Query that can figure out the cheapest supplier of a specific product based on the average cost per item (cost/quantity) of the latest 3 entires in the expenses table (based on date).
That would require a query that works with user variables to generate a ranking based on date. And only select the three last dates.
Query
SELECT
*
FROM (
SELECT
*
, CASE
WHEN @supplier = supplier
THEN @rank := @rank + 1
ELSE @rank := 1
END
AS rank
, @supplier := supplier
FROM
Expenses
CROSS JOIN (
SELECT
@supplier := NULL
, @rank := 0
)
AS
init_user_params
WHERE
product_id = 1
ORDER BY
supplier ASC
, DATE DESC
)
AS Expenses_ranked
WHERE
Expenses_ranked.rank <= 3
Result
id product_id cost quantity supplier date @supplier := NULL @rank := 0 rank @supplier := supplier
------ ---------- ------ -------- -------- ---------- ----------------- ---------- ------ -----------------------
4 1 3.98 2 1 2017-09-22 (NULL) 0 1 1
3 1 2.50 1 1 2017-09-20 (NULL) 0 2 1
1 1 2.99 1 1 2017-09-05 (NULL) 0 3 1
6 1 8.00 2 2 2017-09-27 (NULL) 0 1 2
5 1 4.00 1 2 2017-09-25 (NULL) 0 2 2
2 1 3.00 2 2 2017-09-10 (NULL) 0 3 2
Using that results to generate a avg list per supplier.
Query
SELECT
Expenses_ranked.supplier
, AVG(Expenses_ranked.cost / Expenses_ranked.quantity) AS AVG
FROM (
SELECT
*
, CASE
WHEN @supplier = supplier
THEN @rank := @rank + 1
ELSE @rank := 1
END
AS rank
, @supplier := supplier
FROM
Expenses
CROSS JOIN (
SELECT
@supplier := NULL
, @rank := 0
)
AS
init_user_params
WHERE
product_id = 1
ORDER BY
supplier ASC
, DATE DESC
)
AS Expenses_ranked
WHERE
Expenses_ranked.rank <= 3
GROUP BY
Expenses_ranked.supplier
Result
supplier avg
-------- --------------
1 2.4933333333
2 3.1666666667
Now we can use a simple ORDER BY [] ASC LIMIT 1 to get the cheapest supplier
Query
SELECT
Expenses_ranked_avg.supplier AS cheapest_supplier
FROM (
SELECT
Expenses_ranked.supplier
, AVG(Expenses_ranked.cost / Expenses_ranked.quantity) AS AVG
FROM (
SELECT
*
, CASE
WHEN @supplier = supplier
THEN @rank := @rank + 1
ELSE @rank := 1
END
AS rank
, @supplier := supplier
FROM
Expenses
CROSS JOIN (
SELECT
@supplier := NULL
, @rank := 0
)
AS
init_user_params
WHERE
product_id = 1
ORDER BY
supplier ASC
, DATE DESC
)
AS
Expenses_ranked
WHERE
Expenses_ranked.rank <= 3
GROUP BY
Expenses_ranked.supplier
)
AS Expenses_ranked_avg
ORDER BY
Expenses_ranked_avg.avg ASC
LIMIT 1
Result
cheapest_supplier
-------------------
1
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