如何比较SQL中同一列中的两个日期

Question

我必须比较两个日期，它们在表的一列中，我需要这个比较来查找我所需要的特定日期之前和之后的日期，我还必须在三个不同的列中显示它们。

我写了这段代码，但完全错了：

CREATE VIEW
AS
SELECT (CASE 
        WHEN T1.BuyDate > T2.BuyDate THEN T1.BuyDate END)
AS PreviousBuyDate, T1.ItemName, T1.BuyDate,  
(CASE 
        WHEN T1.BuyDate > T2.BuyDate THEN T1.BuyDate END)
AS  NextDate
FROM FoodSara_tbl T1 , FoodSara_tbl T2
GO

投入：

|ItemName | BuyDate | ItemOrigin |
|---------|---------|------------|
| cake    |2020-10-2| UK         |
| coca    |2020-5-2 | US         |
| cake    |2019-10-6| UK         |
| coca    |2020-12-2| US         |

输出：

|PreviousDate | ItemName | BuyDate |NextDate |
|-------------|----------|---------|---------|
| NULL        |cake      |2019-10-6|2020-10-2|
| NULL        |coca      |2020-5-2 |2020-12-2|
|2019-10-6    |cake      |2020-10-2| NULL    |
| 2020-5-2    |coca      |2020-12-2| NULL    |

PS:我得安排一次约会。

Answer 1

用LAG函数试试：

select LAG(BuyDate,1) OVER (PARTITION BY ItemName ORDER BY BuyDate asc) previous_date 
     , ItemName
     , BuyDate
     , LAG(BuyDate,1) OVER (PARTITION BY ItemName  ORDER BY BuyDate desc) next_date
from FoodSara_tbl

参见最终结果：木琴

或

使用LAG和LEAD函数：

select LAG(BuyDate,1) OVER (PARTITION BY ItemName order by BuyDate) previous_date 
     , ItemName
     , BuyDate
     , LEAD(BuyDate,1) OVER (PARTITION BY ItemName order by BuyDate) next_date
from FoodSara_tbl

查看最终结果；木琴