Question

我有三个表：入侵，警报和CCTV，并且我想使用case语句来获取对象的registration_number及其位置，具体取决于它是CCTV还是触发了入侵事件的警报。入侵表。

这是入侵表：

这是警报表：

CCTV表类似于警报表，这是我的代码：

SELECT
    ALARM_ID
,   CCTV_ID
,   CASE
        WHEN
            ALARM_ID    IS  NULL
        AND CCTV_ID IS  NOT NULL
        THEN
            (
                SELECT
                    REGISTRATION_NUMBER, LOCATION
                FROM
                    REMOTE_SECURITY.ALARMS
                WHERE
                    REMOTE_SECURITY.INTRUSIONS.ALARM_ID =   REMOTE_SECURITY.ALARMS.ALARM_ID
            )
        WHEN
            ALARM_ID    IS  NOT NULL
        AND CCTV_ID IS  NULL
        THEN
            (
                SELECT
                    REGISTRATION_NUMBER, LOCATION
                FROM
                    REMOTE_SECURITY.CCTVS
                WHERE
                    REMOTE_SECURITY.INTRUSIONS.cctv_id  =   REMOTE_SECURITY.CCTVS.CCTV_ID
            )
        ELSE
            'not running'
    END
FROM
    REMOTE_SECURITY.INTRUSIONS

我希望最终输出是一个包含两列的表：Location和Registration_number。（我不需要知道它是闭路电视还是闹钟）

任何帮助将不胜感激！

Answer 1

那为什么不做：

SELECT location, registration_number FROM alarms
  WHERE alarm_id IN (SELECT alarm_id FROM intrusions)
UNION
SELECT location, registration_number FROM cctvs
  WHERE cctv_id IN (SELECT cctv_id FROM intrusions)

这应该基本上与您的代码等效，只是更简洁，更接近您的意图

如果您确实想从intrusions表中获取信息，则可能是这样的：

SELECT intrusion_id, datetime_occurred,
  COALESCE(a.location, c.location) AS location,
  COALESCE(a.registration_number, c.registration_number) AS registration_number
FROM intrusions i
  LEFT JOIN alarms a ON i.alarm_id = a.alarm_id
  LEFT JOIN cctvs c ON i.cctv_id = c.cctv_id

不确定要怎么做才能得到语法错误，这样做对我有用：

create temp table intrusions (intrusion_id serial primary key, datetime_occurred timestamp, alarm_id int, cctv_id int);
create temp table alarms (alarm_id serial primary key, location text, registration_number text);
create temp table cctvs (cctv_id serial primary key, location text, registration_number text);

如果您在问题中包含此类代码，将很有帮助！如果使其他人更容易获得更多答案……

postgresql case语句获取另一个列值

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