SQL - LEAD() 函数

Posted 2023-03-31

【中文标题】SQL - LEAD() 函数

【英文标题】：SQL - LEAD() Function

【发布时间】：2020-08-24 20:58:54

【问题描述】：

我尝试使用以下代码：

 SELECT SkillTargetID, DateTime,
    lead(EventName,1) over (partition by EventName order by DateTime) as next_eventname,
    lead(ReasonCode,1) over (partition by ReasonCode order by DateTime) as next_ReasonCode
 FROM ucce1_awdb.dbo.Agent_State_Trace ast
 WHERE 
 EventName = 3 and
 ReasonCode= 0 and
 next_eventname = 0 and
 next_ReasonCode = 114

但由于next_eventname 和next_ReasonCode，我无法执行该查询。有人可以告诉我应该如何处理吗？

【问题讨论】：

lead(EventName,1) over (partition by EventName 没用，eventname 在同一个 PARTITION 中永远不会改变（目前它与where event_name = 0 非常相似）。 next_reason_cde 也一样。你能展示一些示例数据和预期结果吗？

【参考方案1】：

你需要一个子查询：

SELECT SkillTargetID, DateTime, next_eventname, next_ReasonCode
FROM (SELECT ast.*
             lead(EventName) over (partition by EventName order by DateTime) as next_eventname,
             lead(ReasonCode) over (partition by ReasonCode order by DateTime) as next_ReasonCode
      FROM ucce1_awdb.dbo.Agent_State_Trace ast
     ) ast
WHERE EventName = 3 and
      ReasonCode= 0 and
      next_eventname = 0 and
      next_ReasonCode = 114

【参考方案2】：

您不能在where 子句中使用select 子句中定义的别名...并且您不能在where 子句中使用窗口函数。您需要将过滤移至外部查询：

SELECT *
FROM (
    SELECT 
        SkillTargetID, 
        DateTime,
        lead(EventName,1) 
            over(partition by EventName order by DateTime) as next_eventname,
        lead(ReasonCode,1) 
            over(partition by ReasonCode order by DateTime) as next_ReasonCode
    FROM ucce1_awdb.dbo.Agent_State_Trace ast
) t
WHERE 
    EventName = 3 
    AND ReasonCode= 0
    AND next_eventname = 0 
    AND next_ReasonCode = 114

请注意，我将所有过滤器移到了外部查询（不仅是那些适用于窗口计算的过滤器）：那是因为您（可能！）需要窗口函数来查看整个数据集，在应用过滤器之前。