将前一个值设置为 NULL 用于 LEAD 功能

Posted 2023-03-31

【中文标题】将前一个值设置为 NULL 用于 LEAD 功能

【英文标题】：Set previous value as NULL for LEAD function

【发布时间】：2019-11-20 09:32:44

【问题描述】：

我有以下查询来获取具有 EmployeeId = 的员工的位置历史记录：

SELECT
   e.EmployeeId,
   el.EmployeeLocationId,
   oe.OfficeExtensionId,
   oe.Label,
   [of].OfficeId as 'OfficeId',
   [of].Name as 'Office Name',
   el.LocationDate AS LocationStartDate,
   LEAD(el.LocationDate) OVER ( 
ORDER BY
   el.LocationDate) AS LocationEndDate 
FROM
   Employee_Location el 
   LEFT JOIN
      Employee e 
      ON el.EmployeeId = e.EmployeeId 
   LEFT JOIN
      OfficeExtension oe 
      ON el.OfficeExtensionId = oe.OfficeExtensionId 
   INNER JOIN
      Office [of] 
      ON oe.OfficeId = [of].OfficeId 
WHERE
   el.EmployeeId = 5625

这是我的输出：

EmployeeId  EmployeeLocationId  OfficeExtensionId   OfficeExtensionId   Label      LocationDateStart           LocationDateEnd
5625        6265                156                 156                 Romania    NULL                        2019-04-09 08:15:37.9583552
5625        6265                156                 156                 Romania    2019-04-09 08:15:37.9583552 2019-07-02 11:00:24.4420116
5625        6390                83                  83                  Spain      2019-07-02 11:00:24.4420116 NULL

我想在没有 LocationDateStart 的地方将 NULL 值设置为 LocationDateEnd，所以我的预期输出应该如下所示：

EmployeeId  EmployeeLocationId  OfficeExtensionId   OfficeExtensionId   Label      LocationDateStart           LocationDateEnd
5625        6265                156                 156                 Italy      NULL                        NULL
5625        6265                156                 156                 Romania    2019-04-09 08:15:37.9583552 2019-07-02 11:00:24.4420116
5625        6390                83                  83                  Spain      2019-07-02 11:00:24.4420116 NULL

【问题讨论】：

一个简单的CASE WHEN LocationDate IS NOT NULL THEN LEAD()... END 就可以了。

【参考方案1】：

只需将LEAD() 函数包装在CASE 表达式中：

el.LocationDate AS LocationStartDate,
CASE WHEN el.LocationDate IS NOT NULL THEN LEAD(el.LocationDate)
                                           OVER (ORDER BY el.LocationDate)
END AS LocationEndDate,