内部联接不必要地删除了一些行

Question

我有 3 个表这样定义

CREATE TABLE participants(
    id SERIAL PRIMARY KEY,
    Name TEXT NOT NULL,
    Title TEXT NOT NULL
);

CREATE TABLE meetings (
    id SERIAL PRIMARY KEY,
    Subject TEXT NOT NULL,
    Organizer TEXT NOT NULL,
    StartTime TIMESTAMP NOT NULL,
    EndTime TIMESTAMP NOT NULL
);

CREATE TABLE meetings_participants(
    meeting_id int not null,
    participant_id int not null,
    primary key (meeting_id, participant_id),
    foreign key(meeting_id) references meetings(id),
    foreign key(participant_id) references participants(id)
);

我想查找今天与参与者一起举行的会议。当我运行这个查询时，我基本上得到了它们

SELECT * from meetings 
INNER JOIN meetings_participants ON meetings.id = meetings_participants.meeting_id 
INNER JOIN participants ON meetings_participants.participant_id = participants.id 
WHERE starttime::date = NOW()::date;

问题是此查询会丢弃还没有参与者的会议，我仍然希望将它们包含在我的查询结果中。我怎样才能修改我的查询以像那样工作？

Answer 1

您需要 LEFT JOIN 而不是 INNER。使用 ::date 铸造你是在暗示你只对今天发生的事情感兴趣，不管它是否已经结束。您仍然应该在查询中包含 EndTime，考虑到可能会有跨越数天的会议：[=14=]

SELECT * from meetings 
left join meetings_participants on meetings.id = meetings_participants.meeting_id 
left join participants on meetings_participants.participant_id = participants.id 
WHERE starttime::date <= NOW()::date and endtime::date >= NOW()::date ;

DBFiddle demo 这里

编辑：参与者的姓名和头衔作为 JSON 数组：

SELECT id, subject, organizer, starttime, endtime, jsonb_pretty(tmp.participants)
from meetings m
         left join lateral (
    select jsonb_agg(row_to_json(tp)) as participants
    from (select p.name, p.title
          from meetings_participants mp
                   inner join participants p on mp.participant_id = p.id
          where mp.meeting_id = m.id
         ) tp
    ) tmp on true
WHERE starttime::date <= NOW()::date
  and endtime::date >= NOW()::date;

DBFiddle demo for participants added as JSON

Answer 2

您没有提到您是希望每个参与者在单独的行中还是作为一个集合（例如，逗号分隔列表）。如果是前者，则将内部连接更改为左连接。对于后一种情况，您可以：

SELECT meetings.*, (
    SELECT string_agg(participants.name, ', ')
    FROM meetings_participants
    JOIN participants ON meetings_participants.participant_id = participants.id
    WHERE meetings_participants.meeting_id = meetings.id
) AS participants_list
FROM meetings 
WHERE starttime::date = current_date

内部联接不必要地删除了一些行

Inner join removes some rows unnecessarily

sql

postgresql

join