將列名轉置為行並將值放在單獨的列中

Question

我有下表，您也可以在此處的SQL fiddle找到：

CREATE TABLE Flows (
    Product TEXT,
    Inbound_Date DATE,
    Outbound_Date DATE,
    Return_Date DATE,
    Quantity VARCHAR(255)
);

INSERT INTO Flows
(Product, Inbound_Date, Outbound_Date, Return_Date, Quantity)
VALUES 

("Product A", "2019-01-01", NULL, NULL, "400"),
("Product A", NULL, "2019-05-08", NULL, "200"),
("Product A", NULL, NULL, "2019-06-25", "600"),

("Product B", "2019-03-08", NULL, NULL, "380"),
("Product B", NULL, "2019-03-15", NULL, "120"),
("Product B", NULL, NULL, "2019-04-17", "610");

我使用以下 SQL 從表中獲取值：

SELECT Product, Inbound_Date, Outbound_Date, Return_Date, sum(Quantity)
FROM Flows
GROUP BY 1,2,3,4;

到目前為止，所有這些都運行良好。

---

但是，現在我想實現日期顯示在名為FLow_Date列中。
結果應如下所示：

Product          Date_Type            Flow_Date
Product A        Inbound_Date        2019-01-01
Product A        Oubound_Date        2019-05-08
Product A        Return_Date         2019-06-25
Product B        Inbound_Date        2019-03-08
Product B        Outbound_Date       2019-03-15
Product B        Return_Date         2019-04-17

我需要在代碼中更改什么才能使其正常工作？

Answer 1

您可以使用coalesce()和case表達式：

select
    product,
    case 
        when inbound_date is null  then 'inbound_date' 
        when outbound_date is null then 'outbound_date'
        when return_date is null   then 'return_date'
    end date_type,
    coalesce(inbound_date, outbound_date, return_date) flow_date
from mytable

如果你想在你的聚合查詢中使用這個：

select
    product,
    case 
        when inbound_date is null  then 'inbound_date' 
        when outbound_date is null then 'outbound_date'
        when return_date is null   then 'return_date'
    end date_type,
    coalesce(inbound_date, outbound_date, return_date) flow_date,
    sum(quantity) sum_quantity
from mytable
group by 
    product,
    date_type,
    flow_date

Answer 2

我認為你只需要case表達式：

select f.product,
       (case when inbound_date is not null then 'inbound_date' 
             when outbound_date is not null then 'outbound_date'
             when return_date is not null then 'return_date'
        end) date_type,
       (case when inbound_date is not null then inbound_date
             when outbound_date is not null then outbound_date
             when return_date is not null then return_date
        end) as flow_date
from flows f;

如果每行可以有多個非NULL值，這會變得更加復雜。

在這種情況下， union all是一個足夠簡單的方法

select fd.*
from ((select f.product, 'inbound_date' as date_type, inbound_date as flow_date
       from flows f
      ) union all
      (select f.product, 'outbound_date' as date_type, outbound_date as flow_date
       from flows f
      ) union all
      (select f.product, 'return_date' as date_type, return_date as flow_date
       from flows f
      )
     ) fd
where flow_date is not null;

這不一定是最有效的方法（在這種情況下），但效果很好。 如果flows確實是一種觀點，那么替代方案可能是首選。