【问题标题】:How to find the average of a subset of values from a table in PostgreSQL?如何从 PostgreSQL 中的表中找到值子集的平均值?
【发布时间】:2021-06-04 07:41:03
【问题描述】:

原谅我,但我是 PostgreSQL 新手,我的任务是更新某些表中的某些字段。一个特定的字段是平均决策时间,如下所示:

CASE WHEN COUNT(tdrm.dbid) > 0
THEN TO_CHAR((AVG(tdrm.total_processing_time) || ' millisecond')::interval, 'MI:SS.MS')
ELSE '00:00.000'
END AS average_decision_time

COUNT(tdrm.dbid)items_seen。这个逻辑的问题是,我们希望从平均值中排除中止标志等于“AF_ABORT”的项目的总处理时间。

这就是我想要做的:

CASE WHEN COUNT(tdrm.dbid) > 0
THEN TO_CHAR((AVG(COUNT(CASE WHEN tdrm.tdr_abort_flag!=AF_ABORT THEN tdrm.total_processing_time END)) || ' millisecond')::interval, 'MI:SS.MS')
ELSE '00:00.000'
END AS average_decision_time

但我收到以下错误:

错误:聚合函数调用不能嵌套 第 64 行:THEN TO_CHAR((AVG(COUNT(CASE WHEN tdrm.tdr_abort_flag!=A...

我是在正确的轨道上还是有更简单的方法?

完整的SQL如下:

SELECT s.*,
CASE WHEN agent_event.event_code = 'data_download' THEN 'DL'
WHEN agent_event.event_code = 'mode' THEN 'Mode'
ELSE agent_event.event_code
END AS userAction
FROM
(
WITH report_constants AS (
-- Decisions from DetectionReport.h
SELECT
0::int as AD_UNKNOWN,
1::int as AD_ALARM,
2::int as AD_CLEAR,
-- Flags from DetectionReport.h
0::int as AF_UNKNOWN,
1::int as AF_ABORT,
2::int as AF_SUCCESS,
-- UI values for Decisions are DIFFERENT
0::int as UI_AD_ALL,
1::int as UI_AD_CLEAR,
2::int as UI_AD_ALARM,
3::int as UI_AD_UNKNOWN,
--
0::int as AGENT_TYPE_SCANNER,
1::int as AGENT_TYPE_OSR,
2::int as AGENT_TYPE_DIVERTER,
3::int as AGENT_TYPE_TIP,
4::int as AGENT_TYPE_SEARCH,
-- Operation Mode from Module.h
0::int as OPERATION_MODE_UNKNOWN,
1::int as OPERATION_MODE_SCAN,
2::int as OPERATION_MODE_OTHER
)
SELECT
nss_user.username AS user_name,
reg_login.action_time AS login_action_time,
reg_logout.action_time AS logout_action_time,
to_char(reg_login.action_time, 'MM-DD-YYYY') AS login_date,
to_char(reg_login.action_time, 'HH24:MI:SS') AS login_time,
CASE WHEN reg_logout.action_time IS NULL THEN '' ELSE 
to_char(reg_logout.action_time, 'MM-DD-YYYY') END AS logout_date,
CASE WHEN reg_logout.action_time IS NULL THEN '' ELSE 
to_char(reg_logout.action_time, 'HH24:MI:SS') END AS logout_time,
CASE WHEN user_level.name LIKE 'Level %' THEN SUBSTRING(user_level.name from 7) ELSE user_level.name END AS userAccess,
COUNT(tdrm.dbid) AS items_seen,
CASE WHEN COUNT(tdrm.dbid) > 0
THEN ROUND(100.0 * COUNT(CASE WHEN tdrm.tdr_abort_flag=AF_SUCCESS
  AND tdrm.tdr_alarm_decision=AD_CLEAR THEN 1 END) / COUNT(tdrm.dbid), 2)
ELSE 0.00
END AS clear_rate,
COUNT(CASE WHEN (tdrm.tdr_abort_flag=AF_SUCCESS 
AND tdrm.tdr_alarm_decision=AD_UNKNOWN) 
  OR tdrm.tdr_abort_flag=AF_ABORT THEN 1 END) AS operator_timeout,
CASE WHEN COUNT(tdrm.dbid) > 0
THEN ROUND(100.0 * COUNT(CASE WHEN tdrm.tdr_abort_flag=AF_SUCCESS
  AND tdrm.tdr_alarm_decision=AD_ALARM THEN 1 END) / COUNT(tdrm.dbid), 2)
ELSE 0.00
END AS suspect_rate,
CASE WHEN COUNT(tdrm.dbid) > 0
THEN ROUND(100.0 * COUNT(CASE WHEN 
  (tdrm.tdr_abort_flag=AF_SUCCESS AND tdrm.tdr_alarm_decision=AD_UNKNOWN) 
    OR tdrm.tdr_abort_flag=AF_ABORT THEN 1 END) / COUNT(tdrm.dbid), 2)
ELSE 0.00
END AS operatorNoDecisionRate,
CASE WHEN COUNT(tdrm.dbid) > 0
THEN TO_CHAR((AVG(CASE WHEN tdrm.tdr_abort_flag!=AF_ABORT THEN (tdrm.total_processing_time) END) || ' millisecond')::interval, 'MI:SS.MS')
ELSE '00:00.000'
END AS average_decision_time
v2_module.dbid AS v2_gem_dbid
FROM report_constants CROSS JOIN auth_event
INNER JOIN registration_event AS reg_login
ON reg_login.credential_id=auth_event.credential_id
AND reg_login.event_type=3
LEFT OUTER JOIN registration_event AS reg_logout
ON reg_logout.credential_id=auth_event.credential_id
AND reg_logout.event_type=4
INNER JOIN nss_user ON nss_user.dbid=auth_event.nss_user_dbid
INNER JOIN user_level ON user_level.dbid=nss_user.user_level_dbid
LEFT OUTER JOIN bag_tdr ON nss_user.dbid=bag_tdr.author_user_dbid
AND (item_tdr.agent_type=AGENT_TYPE_OSR OR 
item_tdr.agent_type=AGENT_TYPE_SEARCH)
AND item_tdr.author_credential_id=auth_event.credential_id
LEFT OUTER JOIN v2_module AS tdrm ON 
item_tdr.v2_module_dbid=tdrm.dbid 
LEFT OUTER JOIN v2_general_equipment_module
ON v2_general_equipment_module.dbid=reg_login.v2_gem_dbid
WHERE auth_event.credential_id IS NOT NULL
AND auth_event.auth_event_type=1
AND ($P{userid} = 'ALL' OR $P{userid} = nss_user.username)
AND item_tdr.created_date >= $P{fromdate}
AND item_tdr.created_date <= $P{todate}
AND v2_module.operation_mode != OPERATION_MODE_OTHER
GROUP BY nss_user.username, user_level.name, reg_login.agent_type, 
reg_login.action_time, reg_logout.action_time, 
v2_module.dbid
) s
LEFT OUTER JOIN agent_event
ON s.v2_dbid=agent_event.v2_dbid
AND agent_event.event_timestamp >= s.login_action_time
AND (s.logout_action_time IS NULL OR agent_event.event_timestamp <= s.logout_action_time)
ORDER BY s.login_action_time

【问题讨论】:

    标签: sql postgresql average aggregate-filter


    【解决方案1】:

    对于中止标志等于“AF_ABORT”的项目,我们希望从平均值中排除总处理时间。

    CASE WHEN count(tdrm.dbid) > 0
       THEN to_char(avg(tdrm.total_processing_time)
                       FILTER (WHERE tdrm.tdr_abort_flag IS DISTINCT FROM 'AF_ABORT') -- ①, ②
                  * interval '1 millisecond'  -- ③
                  , 'MI:SS.MS')
       ELSE '00:00.000'
    END AS average_decision_time
    

    ① 实现过滤器的关键元素是聚合FILTER 子句。见:

    ②如果tdrm.tdr_abort_flag可以为NULL(缺少信息),我们需要tdrm.tdr_abort_flag IS DISTINCT FROM 'AF_ABORT'。否则我们可以简化为tdrm.tdr_abort_flag &lt;&gt; 'AF_ABORT'

    ③ 乘法比串联加强制转换要快得多。

    但是像这样添加FILTER之后,表达式毕竟可以产生NULL值。你的要求是模糊的。你可能真的想要:

    total_processing_time 的平均值,其中tdr_abort_flag &lt;&gt; 'AF_ABORT'。如果结果为NULL,则默认为0

    COALESCE(to_char(avg(tdrm.total_processing_time) FILTER (WHERE tdrm.tdr_abort_flag <> 'AF_ABORT')
                   * interval '1 millisecond'
                   , 'MI:SS.MS')
           , '00:00.000') AS average_decision_time
    

    或者:

    total_processing_time 的平均值,其中tdr_abort_flag &lt;&gt; 'AF_ABORT'。但前提是count(tdrm.dbid) &gt; 0。如果由于任何原因结果为 NULL,则默认为 0

    CASE WHEN count(tdrm.dbid) > 0
       THEN COALESCE(to_char(avg(tdrm.total_processing_time) FILTER (WHERE tdrm.tdr_abort_flag <> 'AF_ABORT')
                           * interval '1 millisecond'
                           , 'MI:SS.MS')
                   , '00:00.000')
       ELSE '00:00.000'
    END AS average_decision_time
    

    或者:

    total_processing_time 的平均值,其中tdr_abort_flag &lt;&gt; 'AF_ABORT'。但前提是count(tdrm.dbid) &gt; 0 在哪里tdr_abort_flag &lt;&gt; 'AF_ABORT'。否则默认为0

    CASE WHEN count(tdrm.dbid) FILTER (WHERE tdrm.tdr_abort_flag <> 'AF_ABORT') > 0
       THEN to_char(avg(tdrm.total_processing_time) FILTER (WHERE tdrm.tdr_abort_flag <> 'AF_ABORT')
                  * interval '1 millisecond'
                  , 'MI:SS.MS')
       ELSE '00:00.000'
    END AS average_decision_time
    

    您提到您是“SQL 新手”。让我补充一下:问题的清晰定义是> 50%的解决方案。在许多领域都是如此,但对于 SQL 来说肯定是这样。

    【讨论】:

      【解决方案2】:

      您应该能够通过一个简单的子查询来实现这一点,该子查询从表中重新选择并在计算平均值时使用WHERE 子句过滤掉中止的记录。像这样的:

          CASE WHEN COUNT(tdrm.dbid) > 0
              THEN TO_CHAR((SELECT (AVG(CASE WHEN tdrm_subq.tdr_abort_flag != AF_ABORT
                                             THEN (tdrm_subq.total_processing_time)
                                        END) || ' millisecond')
                            FROM v2_module tdrm_subq, report_constants
                            WHERE tdr_abort_flag != AF_ABORT)::interval,
                           'MI:SS.MS')
              ELSE '00:00.000'
          END AS average_decision_time
      

      在这里演示:https://rextester.com/NSQN12214

      【讨论】:

      • 好的,我已经添加了完整的 SQL。我不相信这会起作用,因为我们确实希望在其他字段中没有设置 AF_ABORT 标志的项目
      • 感谢您提供该信息 - 现在更清晰了。我已经相应地修改了答案......
      【解决方案3】:

      除了子查询之外,应该提到它是 Window Functions 的确切情况。

      【讨论】:

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