SQL Teradata 时的嵌套案例答案

【问题标题】：Nested Case when SQL TeradataSQL Teradata 时的嵌套案例
【发布时间】：2021-05-04 19:40:22
【问题描述】：

请我有以下数据，我需要计算每个 ID 的 Current_date 和 Max Date 之间的日期差异，仅限 Active Status，日期差异的结果将位于最大日期旁边，并且其他记录返回 NULL。

|ID  |Date  |Status   |
|----+------|---------|
|A   |1-Apr |Active   |
|A   |15-Apr|Active   |
|B   |1-Mar |Suspended|
|B   |15-Mar|Deactive |
|C   |1-Jan |Active   |
|C   |15-Jan|Active   |

我尝试使用以下查询，但结果与每个日期重复

SELECT
    ID,
    Date, 
    CASE WHEN STATUS = 'Active' THEN
    CASE WHEN Date = MAX(Date) OVER (PARTITION BY ID)
         THEN CURRENT_DATE - MAX(Date) OVER (PARTITION BY ID) ELSE NULL END
ELSE NULL END AS Duration
FROM cte
ORDER BY ID, Date;

但我需要结果如下所示

|ID  |Date  |Status   |Duration|
|----+------|---------|--------|
|A   |1-Apr |Active   |NULL    |
|A   |15-Apr|Active   |19      |
|B   |1-Mar |Suspended|NULL    |
|B   |15-Mar|Deactive |NULL    |
|C   |1-Jan |Active   |NULL    |
|C   |15-Jan|Active   |109     |

【问题讨论】：

也许是愚蠢的问题，但为什么不将两者与 AND => CASE WHEN STATUS = 'Active' AND Date = MAX(Date) OVER (PARTITION BY ID) THEN CURRENT_DATE - MAX(Date) OVER ( PARTITION BY ID) ELSE NULL END AS Duration
@hhoeck 我尝试使用 AND 但它也会在每个日期重复结果
SELECT ID, DATE, Status, CASE WHEN STATUS = 'Active' AND RNUM = 1 THEN CURRENT_DATE - DATE ELSE NULL END AS Duration FROM (SELECT ID, DATE, Status, ROW_NUMBER() OVER (PARTITION BY Status ORDER BY DATE DESC) AS RNUM FROM CTE) CTE2 ORDER BY ID, DATE
除非您每个 id/date 有多个行，否则 hhoeck 的建议应该按预期工作。

标签： sql nested teradata

【解决方案1】：

如果您只想在带有“ACTIVE”的最新行上使用此功能，请添加 row_number():

SELECT ID, Date, 
       (CASE WHEN STATUS = 'Active' AND
                  ROW_NUMBER() OVER (PARTITION BY ID ORDER BY DATE DESC) = 1
             THEN CURRENT_DATE - Date
        END) as Duration
FROM cte
ORDER BY ID, Date;

请注意，您的代码看起来应该可以工作，除非id 的最近日期重复。然而，这个版本更简单一些，避免了嵌套的CASE 表达式和对MAX() 的不必要的第二次调用。

【讨论】：

【解决方案2】：

SELECT ID,
       DATE,
       Status,
       CASE
         WHEN STATUS = 'Active' AND RNUM = 1 THEN CURRENT_DATE - DATE
         ELSE NULL
       END AS Duration
FROM (SELECT ID,
             DATE,
             Status,
             ROW_NUMBER() OVER (PARTITION BY ID,Status ORDER BY DATE DESC) AS RNUM
      FROM CTE) CTE2
ORDER BY ID,
         DATE;

【讨论】：