我正在研究基于PHP和MySQL的系统来组织餐馆的产品和费用。
我将数据整理成四个表格。
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，根据费用表（基于日期）中最新3个项目的平均每项成本（成本/数量），找出最便宜的特定产品供应商。< / p>
这是我想要计算的内容：
供应商1最后3个条目 - 每单位成本：2.99,2.50和1.99。平均值= 2.49
供应商2最后3个条目 - 每单位成本：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（百威啤酒）最便宜的选择。
到目前为止，我已经尝试了这一点，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;
此查询的输出距离我想要解决的问题还有很长的路要走：（：
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
我根据样本数据寻找的输出是：
cheapest_supplier
1
答案 0 :(得分：4)
我想编写一个MYSQL查询，可以根据每个平均成本计算出最便宜的特定产品供应商 费用表中最新3个项目的项目（成本/数量） （根据日期而定）。
这将需要一个与用户变量一起使用的查询，以根据日期生成排名。 并且只选择最后三个日期。
<强>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
<强>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。
<强>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
<强>结果
supplier avg
-------- --------------
1 2.4933333333
2 3.1666666667
现在我们可以使用简单的ORDER BY [] ASC LIMIT 1来获得最便宜的供应商
<强>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
<强>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。 直接过滤掉排名。
<强>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
<强>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，使用此结果集找到最便宜的供应商很容易。
<强>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
<强>结果
cheapest_supplier
-------------------
1
答案 1 :(得分：1)
E.g：
SELECT x.product_id
, x.supplier_id
, x.date
, ROUND(AVG(y.cost/y.quantity),2) total_cost
FROM expenses x
JOIN expenses y
ON y.product_id = x.product_id
AND y.supplier_id = x.supplier_id
AND y.date <= x.date
GROUP
BY product_id
, supplier_id
, date
HAVING COUNT(*) = 3;