去除重复行，只保留最新的工作日志条目

Question

我创建了这个查询，它可以工作。问题是它显示了每个工作日志的行。我只需要每个事件的最新工作日志。

SELECT DISTINCT
`HPD:Help Desk`.`Incident Number` AS Incident_Number,
`HPD:Help Desk`.`Status` AS Status,
`HPD:Help Desk`.`Priority` AS Priority,
`HPD:Help Desk`.`Assigned Group` AS Assigned_Group,
`HPD:Help Desk`.`Assignee` AS Assignee,
`HPD:Help Desk`.`Submit Date` AS Submit_Date,
`HPD:Help Desk`.`Last Modified Date` AS Last_Modified_Date,
`HPD:Help Desk`.`Description` AS Description,
`HPD:Help Desk`.`Submitter` AS Submitter,
`HPD:Search-Worklog`.`Worklog Description` AS Worklog_Description,
`HPD:Search-Worklog`.`Work Log ID` AS Work_Log_ID
FROM
`HPD:Help Desk` 
INNER JOIN `HPD:Search-Worklog` on (`HPD:Search-Worklog`.`InstanceId` = `HPD:Help Desk`.`InstanceId`)
WHERE 
(`HPD:Help Desk`.`Status` in ('Assigned','Pending','In Progress')
And `HPD:Help Desk`.`Submit Date` >= $__from/1000 and `HPD:Help Desk`.`Submit Date` <= $__to/1000)
ORDER BY
`HPD:Help Desk`.`Submit Date` ASC
LIMIT 20

我尝试过HPD:Help Desk、HPD:Worklog和HPD:Search-Worklog表单。我还尝试使用CASE语句只显示每个事件的最新/最大工作日志ID，但没有成功。我确定我漏掉了一些明显/简单的东西。

Answer 1

假设使用支持窗口函数的最新版本的MySQL（或其变体），获取“最新”的一种非常有效的方法是使用row_number() over()。在over子句中使用partition by来控制什么会触发行编号重置，并使用order by来控制分区中获得行号1的行。现在，要获取“最新”的需要在worklog表中有一列来实现这个功能 - 我假设有一个时间戳列（worklog_entry_date存在于该表中（worklog_entry_date），您需要替换实际确定“最新”的列）。将当前查询嵌套一层，以便您可以过滤行号为1的情况 - 这些将是“最新”的行：

SELECT
    *
FROM (
    SELECT
          h.`Incident Number` AS Incident_Number
        , h.`Status` AS STATUS
        , h.`Priority` AS Priority
        , h.`Assigned Group` AS Assigned_Group
        , h.`Assignee` AS Assignee
        , h.`Submit Date` AS Submit_Date
        , h.`Last Modified Date` AS Last_Modified_Date
        , h.`Description` AS Description
        , h.`Submitter` AS Submitter
        , w.`Worklog Description` AS Worklog_Description
        , w.`Work Log ID` AS Work_Log_ID
        , row_number() over(partition by h.`Incident Number`
                            order by w.worklog_entry_date DESC) as rn
    FROM `HPD: HELP Desk` h
    INNER JOIN `HPD: Search - Worklog` w ON w.`InstanceId` = h.`InstanceId`
    WHERE h.`Status` IN ('Assigned', 'Pending', 'In Progress')
        AND h.`Submit Date` >= $_ _from / 1000
        AND h.`Submit Date` <= $_ _to / 1000
    ) d
WHERE d.rn = 1
ORDER BY
      d.`Submit Date` ASC 
LIMIT 20

注意：当行编号存在时，您不需要使用select distinct - 一般情况下尽量避免使用它。此外，请自己使用表别名来简化代码。