我正在开发一个基于 PHP 和 MySQL 的系统来组织餐厅的产品和费用。
我将数据组织成四个表。
项目表
id | name
1 | Beer
2 | Vodka
产品表
id | item_id | name
1 | 1 | Budweiser
2 | 1 | Sam Adams
3 | 2 | Smirnoff
4 | 2 | Grey Goose
供应商表
id | name
1 | Supplier 1
2 | Supplier 2
费用表
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
我想编写一个 MYSQL 查询，它可以根据费用表(基于日期)中最新 3 个整体的每件商品的平均成本(成本/数量)找出特定产品的最便宜供应商。
这是我要计算的内容:
供应商 1 最后 3 个条目 - 单位成本:2.99、2.50 和 1.99。平均值 = 2.49
供应商 2 最后 3 个条目 - 单位成本:1.50、4.00 和 4.00。平均值 = 3.16
因此 SQL 应该返回供应商 1 是产品 1(百威)最便宜的选择。
到目前为止我已经尝试过了，但我有点迷茫和困惑:
select * from products
INNER JOIN expenses
ON products.id = expenses.product
AND products.item = '1'
ORDER BY (expenses.cost/expenses.quantity)
LIMIT 3;
这个查询的输出与我想要弄清楚的结果相去甚远:(:
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
我根据示例数据寻找的输出是:
cheapest_supplier
1
最佳答案
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).
一个选项将是一个查询，它与用户变量一起使用以生成基于日期的排名。 并且只选择最后三个日期。
查询
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
结果
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
使用该结果生成每个供应商的平均列表。
查询
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
结果
supplier avg
-------- --------------
1 2.4933333333
2 3.1666666667
现在我们可以使用一个简单的 ORDER BY [] ASC LIMIT 1 来获得最便宜的供应商
查询
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
结果
cheapest_supplier
-------------------
1
更优化的查询。
还可以在 where 语句中声明用户变量。 可以直接过滤掉排名。
查询
SELECT
*
FROM
Expenses
WHERE
(
CASE
WHEN @supplier = supplier
THEN @rank := @rank + 1
ELSE @rank := 1
END
)
AND
(@supplier := supplier )
AND
@rank <= 3
AND
product_id = 1
ORDER BY
supplier ASC
, DATE ASC
结果
id product_id cost quantity supplier date
------ ---------- ------ -------- -------- ------------
1 1 2.99 1 1 2017-09-05
3 1 2.50 1 1 2017-09-20
4 1 3.98 2 1 2017-09-22
2 1 3.00 2 2 2017-09-10
5 1 4.00 1 2 2017-09-25
6 1 8.00 2 2 2017-09-27
现在使用此结果集可以轻松找到最便宜的供应商。
查询
SELECT
Expenses_ranked.supplier AS cheapest_supplier
FROM (
SELECT
*
FROM
Expenses
WHERE
(
CASE
WHEN @supplier = supplier
THEN @rank := @rank + 1
ELSE @rank := 1
END
) IS NOT NULL
AND
(@supplier := supplier ) IS NOT NULL
AND
@rank <= 3
AND
product_id = 1
ORDER BY
supplier ASC
, DATE ASC
)
AS Expenses_ranked
GROUP BY
Expenses_ranked.supplier
ORDER BY
AVG(Expenses_ranked.cost / Expenses_ranked.quantity) ASC
LIMIT 1
结果
cheapest_supplier
-------------------
1
关于MySQL 查询使用多个表中的数据计算最新的平均价格，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/46514359/