Question

SQL Fiddle：

CREATE TABLE Purchasing (
    Event_Type VARCHAR(255),
    Product VARCHAR(255),
    Quantity VARCHAR(255)
);

INSERT INTO Purchasing
(Event_Type, Product, Quantity)
VALUES 
("Offer", "Product_A", "300"),
("Offer", "Product_B", "200"),
("Offer", "Product_C", "500"),
("Offer", "Product_D", "400"),
("Offer", "Product_E", "600"),
("Order", "Product_B", "250"),
("Order", "Product_C", "450");

该表格使用products Event_Type或Offer显示不同Order的购买状态。

如您所见，某些产品具有Event_Type Order，而其他产品具有Event_Type Offer。
现在，我想实现的是，一旦Product具有Event_Type Order的{{1}}中的Quantity取代了Order Quantity中的一个。结果应如下所示：

Offer

到目前为止，如果满足条件，我只能查询数据而不替换值：

Product          Event_Type        Quantity
Product_A          Offer             300
Product_B          Order             250
Prodcut_C          Order             450
Product_D          Offer             400
Product_E          Offer             600

要获得结果，我需要在SQL中进行哪些更改？

Answer 1

您可以GROUP BY Product并在子查询中选择max Event_Type，然后再次与原始表进行联接

SELECT p2.Product,p2.Event_Type,p2.Quantity
  FROM
  (
   SELECT Product, max(Event_Type) AS Event_Type
     FROM Purchasing
    GROUP BY Product
  ) p1
  JOIN Purchasing p2
    ON p2.Product=p1.Product
      AND p2.Event_Type=p1.Event_Type
 ORDER BY p2.Product

Demo

Answer 2

从MySQL 8开始，排名通常是通过窗口函数完成的。我在这里使用MAX OVER，因为'Order' > 'Offer'。

select product, event_type, quantity
from
(
  select p.*, max(event_type) over (partition by product) as best_event_type
  from purchasing p
) ranked
where event_type = best_event_type;

演示：https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=b1337c181fd28c2ac3c1bfa18d60108f

更新：要获取更多值，请改用ROW_NUMBER（或RANK或DENSE_RANK）。例如。类型排名'Accept' >> 'Order' >> 'Offer' >> 'Reject'：

select product, event_type, quantity
from
(
  select p.*, 
    row_number() over (partition by product
                       order by case event_type
                         when  'Accept' then 1
                         when  'Order' then 2
                         when  'Offer' then 3
                         when  'Reject' then 4
                       end) as rn
  from purchasing p
) ranked
where rn = 1;

演示：https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=371817125aff1160f6865babd1f0f3bf

Answer 3

在MySQL 8+中，使用窗口函数：

select p.*
from (select p.*,
             row_number() over (partition by product order by field(Event_Type, 'Order', 'Offer')) as seqnum
      from Purchasing p
     ) p
where seqnum = 1;

在早期版本中，我建议：

select p.*
from Purchasing p
where p.event_type = 'Order' or
      (p.event_type = 'Offer' and
       not exists (select 1 from Purchasing p2 where p2.product = p.product and p2.event_type = 'Order')
      )
order by product;

Here是db <>小提琴。

请注意，这两种方法都适用于仅具有“订单”而没有“要约”的产品。

Answer 4

SELECT x.product
     , COALESCE(y.event_type,x.event_type) event_type
     , COALESCE(y.quantity,x.quantity) quantity
  FROM purchasing x
  LEFT  
  JOIN purchasing y
    ON y.product = x.product
   AND y.event_type = 'order'
 WHERE x.event_type = 'offer'

如果满足条件，则用其他值替换值

4 个答案: