sql - 左外连接的作用类似于内连接

Question

概括

我的目标是找到曾经被分配到任务的每个用户，然后在特定日期范围内生成一些统计数据，并将这些统计数据与原始用户集相关联。当特定用户不存在统计信息时，我希望用户的输出中有一行，但NULL统计信息的值。

我有一个看起来像这样的复杂 SQL 查询（底部的实际查询）：

SELECT
  user_name, changeday, project_name
  sum(hour_delta) AS hours,
FROM ( … ) tasked_users
LEFT OUTER JOIN ( … ) a
ON tasked_users.id=a.assignee_id
WHERE
  (changeday IS NULL) OR (changeday >= … AND changeday <= …)
GROUP BY user_name, changeday, a.project_name
ORDER BY user_name, changeday, a.project_name;

我的愿望是找到大量用户并将他们与a表中的数据进行匹配；当存在没有任何匹配条目的用户时，a我想要空值或0小时数。

不幸的是，此查询仅返回存在于“a”中的用户的行。例如，一组特定的日期返回：

{:user_name=>"Gavin", :hours=>0.0, :changeday=>2013-09-08, :project_name=>"Foo"}
{:user_name=>"Steve", :hours=>1.0, :changeday=>2013-09-08, :project_name=>"Bar"}

虽然不同的日期范围会导致找到不同的用户。tasked_users子查询的内容有 14 个不同的用户 id/name 对。我需要所有这些都在结果中表示出来。

示例查询

如果它有所作为，或者如果您有其他有用的提示来改进查询，这里是完整的查询。

SELECT
  user_name,
  sum(hour_delta) AS hours,
  changeday,
  project_name
FROM (
  SELECT DISTINCT
    users.id,
    users.name AS user_name
  FROM users
  INNER JOIN tasks AS tasks1
  ON users.id=tasks1.assignee_id
) tasked_users
LEFT OUTER JOIN
(
  SELECT
    (
      coalesce(cast(nullif(new_value,'') AS float),0) -
      coalesce(cast(nullif(old_value,'') AS float),0)
    ) AS hour_delta,
    task_id,
    tasks2.assignee_id AS assigned_log,
    fixin_id,
    projects.name AS project_name,
    date_trunc('day',task_log_entries.created_on) AS changeday
  FROM task_log_entries
  INNER JOIN tasks AS tasks2
  ON task_id=tasks2.id
  INNER JOIN fixins
  ON fixins.id=tasks2.fixin_id
  INNER JOIN projects
  ON projects.id=fixins.project_id
  WHERE field_id=18
) a
ON tasked_users.id=a.assigned_log
WHERE
  (changeday IS NULL)
  OR
  (changeday >= '2013-09-08' AND changeday <= '2013-09-08')
GROUP BY user_name, changeday, a.project_name
ORDER BY user_name, changeday, a.project_name;

解释输出

这是EXPLAIN查询的结果，以防有帮助（我不知道如何阅读并得出我需要的内容）：

GroupAggregate  (cost=1116.40..1116.99 rows=13 width=144)"}
  ->  Sort  (cost=1116.40..1116.43 rows=13 width=144)"}
        Sort Key: users.name, (date_trunc('day'::text, task_log_entries.created_on)), projects.name"}
        ->  Hash Left Join  (cost=1024.32..1116.16 rows=13 width=144)"}
              Hash Cond: (users.id = tasks2.assignee_id)"}
              Filter: ((date_trunc('day'::text, task_log_entries.created_on) IS NULL) OR ((date_trunc('day'::text, task_log_entries.created_on) >= '2013-09-08 00:00:00'::timestamp without time zone) AND (date_trunc('day'::text, task_log_entries.created_on) <= '2013-09-08 00:00:00'::timestamp without time zone)))"}
              ->  HashAggregate  (cost=44.07..45.46 rows=139 width=12)"}
                    ->  Hash Join  (cost=5.13..40.09 rows=795 width=12)"}
                          Hash Cond: (tasks1.assignee_id = users.id)"}
                          ->  Seq Scan on tasks tasks1  (cost=0.00..24.01 rows=801 width=4)"}
                          ->  Hash  (cost=3.39..3.39 rows=139 width=12)"}
                                ->  Seq Scan on users  (cost=0.00..3.39 rows=139 width=12)"}
              ->  Hash  (cost=963.51..963.51 rows=1339 width=30)"}
                    ->  Hash Join  (cost=729.23..963.51 rows=1339 width=30)"}
                          Hash Cond: (fixins.project_id = projects.id)"}
                          ->  Hash Join  (cost=727.91..943.79 rows=1339 width=24)"}
                                Hash Cond: (task_log_entries.task_id = tasks2.id)"}
                                ->  Seq Scan on task_log_entries  (cost=0.00..197.46 rows=1339 width=20)"}
                                      Filter: (field_id = 18)"}
                                ->  Hash  (cost=717.90..717.90 rows=801 width=12)"}
                                      ->  Hash Join  (cost=676.87..717.90 rows=801 width=12)"}
                                            Hash Cond: (tasks2.fixin_id = fixins.id)"}
                                            ->  Seq Scan on tasks tasks2  (cost=0.00..24.01 rows=801 width=12)"}
                                            ->  Hash  (cost=589.72..589.72 rows=6972 width=8)"}
                                                  ->  Seq Scan on fixins  (cost=0.00..589.72 rows=6972 width=8)"}
                          ->  Hash  (cost=1.14..1.14 rows=14 width=14)"}
                                ->  Seq Scan on projects  (cost=0.00..1.14 rows=14 width=14)"}

表定义

这是所有涉及的表的描述。我没有修剪它们以删除任何“不相关”的列，因此您可以确保查看是否存在任何模棱两可的列名冲突。

app=> \d task_log_entries
                                     Table "public.task_log_entries"
   Column   |            Type             |                           Modifiers
------------+-----------------------------+---------------------------------------------------------------
 id         | integer                     | not null default nextval('task_log_entries_id_seq'::regclass)
 task_id    | integer                     | not null
 user_id    | integer                     |
 field_id   | integer                     | not null
 created_on | timestamp without time zone | not null default now()
 new_value  | text                        |
 old_value  | text                        |
Indexes:
    "task_log_entries_pkey" PRIMARY KEY, btree (id)
Foreign-key constraints:
    "task_log_entries_field_id_fkey" FOREIGN KEY (field_id) REFERENCES log_fields(id)
    "task_log_entries_task_id_fkey" FOREIGN KEY (task_id) REFERENCES tasks(id) ON DELETE CASCADE
    "task_log_entries_user_id_fkey" FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE SET NULL


app=> \d tasks
                                        Table "public.tasks"
     Column     |            Type             |                      Modifiers
----------------+-----------------------------+-----------------------------------------------------
 id             | integer                     | not null default nextval('fixins_id_seq'::regclass)
 fixin_id       | integer                     | not null
 created_on     | timestamp without time zone | not null default now()
 updated_on     | timestamp without time zone | not null default now()
 name           | character varying(200)      | not null
 description    | text                        |
 blocked_by     | character varying(200)      |
 estimate       | double precision            |
 actual         | double precision            |
 remaining      | double precision            |
 relative_order | integer                     |
 status_id      | integer                     | not null
 assignee_id    | integer                     |
Indexes:
    "tasks_pkey" PRIMARY KEY, btree (id)
Foreign-key constraints:
    "tasks_assignee_id_fkey" FOREIGN KEY (assignee_id) REFERENCES users(id) ON DELETE SET NULL
    "tasks_fixin_id_fkey" FOREIGN KEY (fixin_id) REFERENCES fixins(id) ON DELETE CASCADE
    "tasks_status_id_fkey" FOREIGN KEY (status_id) REFERENCES task_statuses(id)
Referenced by:
    TABLE "task_comments" CONSTRAINT "task_comments_task_id_fkey" FOREIGN KEY (task_id) REFERENCES tasks(id) ON DELETE CASCADE
    TABLE "task_log_entries" CONSTRAINT "task_log_entries_task_id_fkey" FOREIGN KEY (task_id) REFERENCES tasks(id) ON DELETE CASCADE
    TABLE "users_tasks_notifications" CONSTRAINT "users_tasks_notifications_task_id_fkey" FOREIGN KEY (task_id) REFERENCES tasks(id) ON DELETE CASCADE


app=> \d fixins
                                       Table "public.fixins"
     Column     |            Type             |                      Modifiers
----------------+-----------------------------+-----------------------------------------------------
 id             | integer                     | not null default nextval('fixins_id_seq'::regclass)
 project_id     | integer                     | not null
 created_on     | timestamp without time zone | not null default now()
 updated_on     | timestamp without time zone | not null default now()
 name           | character varying(200)      | not null
 description    | text                        | not null
 status_id      | integer                     | not null
 reporter_id    | integer                     |
 assignee_id    | integer                     |
 priority_id    | integer                     | not null
 severity_id    | integer                     | not null
 likelihood_id  | integer                     | not null
 maturity       | integer                     | not null default 0
 version        | character varying(100)      |
 iteration_id   | integer                     |
 relative_order | integer                     |
 kind           | character varying(16)       | not null default 'Bug'::character varying
 specs          | character varying(50)       |
 estimate       | double precision            |
 blocked_by     | character varying(200)      |
 plan_estimate  | double precision            |
 actual         | double precision            |
 remaining      | double precision            |
 promise_date   | date                        |
Indexes:
    "fixins_pkey" PRIMARY KEY, btree (id)
Foreign-key constraints:
    "fixins_assignee_id_fkey" FOREIGN KEY (assignee_id) REFERENCES users(id) ON DELETE SET NULL
    "fixins_iteration_id_fkey" FOREIGN KEY (iteration_id) REFERENCES iterations(id) ON DELETE SET NULL
    "fixins_likelihood_id_fkey" FOREIGN KEY (likelihood_id) REFERENCES likelihoods(id)
    "fixins_priority_id_fkey" FOREIGN KEY (priority_id) REFERENCES priorities(id)
    "fixins_project_id_fkey" FOREIGN KEY (project_id) REFERENCES projects(id)
    "fixins_reporter_id_fkey" FOREIGN KEY (reporter_id) REFERENCES users(id) ON DELETE SET NULL
    "fixins_severity_id_fkey" FOREIGN KEY (severity_id) REFERENCES severities(id)
    "fixins_status_id_fkey" FOREIGN KEY (status_id) REFERENCES statuses(id)
Referenced by:
    TABLE "bug_snapshots" CONSTRAINT "bug_snapshots_fixin_id_fkey" FOREIGN KEY (fixin_id) REFERENCES fixins(id) ON DELETE SET NULL
    TABLE "comments" CONSTRAINT "comments_fixin_id_fkey" FOREIGN KEY (fixin_id) REFERENCES fixins(id) ON DELETE CASCADE
    TABLE "customers_fixins" CONSTRAINT "customers_fixins_fixin_id_fkey" FOREIGN KEY (fixin_id) REFERENCES fixins(id)
    TABLE "fixins_tags" CONSTRAINT "fixins_tags_fixin_id_fkey" FOREIGN KEY (fixin_id) REFERENCES fixins(id) ON DELETE CASCADE
    TABLE "log_entries" CONSTRAINT "log_entries_fixin_id_fkey" FOREIGN KEY (fixin_id) REFERENCES fixins(id) ON DELETE CASCADE
    TABLE "relationships" CONSTRAINT "relationships_fixin1_id_fkey" FOREIGN KEY (fixin1_id) REFERENCES fixins(id) ON DELETE CASCADE
    TABLE "relationships" CONSTRAINT "relationships_fixin2_id_fkey" FOREIGN KEY (fixin2_id) REFERENCES fixins(id) ON DELETE CASCADE
    TABLE "tasks" CONSTRAINT "tasks_fixin_id_fkey" FOREIGN KEY (fixin_id) REFERENCES fixins(id) ON DELETE CASCADE
    TABLE "users_notifications" CONSTRAINT "users_notifications_fixin_id_fkey" FOREIGN KEY (fixin_id) REFERENCES fixins(id) ON DELETE CASCADE
    TABLE "votes" CONSTRAINT "votes_fixin_id_fkey" FOREIGN KEY (fixin_id) REFERENCES fixins(id)


app=> \d projects
                                     Table "public.projects"
     Column     |          Type           |                       Modifiers
----------------+-------------------------+-------------------------------------------------------
 id             | integer                 | not null default nextval('projects_id_seq'::regclass)
 name           | character varying(50)   | not null
 link_name      | character varying(50)   | not null
 pain_threshold | integer                 | not null
 wiki_server    | character varying(100)  |
 wiki_wiki      | character varying(100)  |
 wiki_pattern   | character varying(1000) |
 active         | boolean                 | not null default true
Indexes:
    "projects_pkey" PRIMARY KEY, btree (id)
    "projects_link_name_key" UNIQUE, btree (link_name)
Referenced by:
    TABLE "fixins" CONSTRAINT "fixins_project_id_fkey" FOREIGN KEY (project_id) REFERENCES projects(id)
    TABLE "iterations" CONSTRAINT "iterations_project_id_fkey" FOREIGN KEY (project_id) REFERENCES projects(id) ON DELETE CASCADE
    TABLE "project_preferences" CONSTRAINT "project_preferences_project_id_fkey" FOREIGN KEY (project_id) REFERENCES projects(id)
    TABLE "projects_users_notifications" CONSTRAINT "projects_users_notifications_project_id_fkey" FOREIGN KEY (project_id) REFERENCES projects(id)
    TABLE "releases" CONSTRAINT "releases_project_id_fkey" FOREIGN KEY (project_id) REFERENCES projects(id) ON DELETE CASCADE


app=> \d users
                                 Table "public.users"
  Column  |         Type          |                     Modifiers
----------+-----------------------+----------------------------------------------------
 id       | integer               | not null default nextval('users_id_seq'::regclass)
 name     | character varying(50) | not null
 email    | character varying(50) |
 active   | boolean               | not null default true
 passhash | character varying(40) |
 salt     | character varying(4)  |
Indexes:
    "users_pkey" PRIMARY KEY, btree (id)
Referenced by:
    TABLE "comments" CONSTRAINT "comments_user_id_fkey" FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE SET NULL
    TABLE "fixins" CONSTRAINT "fixins_assignee_id_fkey" FOREIGN KEY (assignee_id) REFERENCES users(id) ON DELETE SET NULL
    TABLE "fixins" CONSTRAINT "fixins_reporter_id_fkey" FOREIGN KEY (reporter_id) REFERENCES users(id) ON DELETE SET NULL
    TABLE "log_entries" CONSTRAINT "log_entries_user_id_fkey" FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE SET NULL
    TABLE "project_preferences" CONSTRAINT "project_preferences_user_id_fkey" FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE
    TABLE "projects_users_notifications" CONSTRAINT "projects_users_notifications_user_id_fkey" FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE
    TABLE "task_comments" CONSTRAINT "task_comments_user_id_fkey" FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE SET NULL
    TABLE "task_log_entries" CONSTRAINT "task_log_entries_user_id_fkey" FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE SET NULL
    TABLE "tasks" CONSTRAINT "tasks_assignee_id_fkey" FOREIGN KEY (assignee_id) REFERENCES users(id) ON DELETE SET NULL
    TABLE "users_notifications" CONSTRAINT "users_notifications_user_id_fkey" FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE
    TABLE "users_tasks_notifications" CONSTRAINT "users_tasks_notifications_user_id_fkey" FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE


app=> \d log_fields
          Table "public.log_fields"
 Column |          Type          | Modifiers
--------+------------------------+-----------
 id     | integer                | not null
 name   | character varying(200) | not null
Indexes:
    "log_fields_pkey" PRIMARY KEY, btree (id)
Referenced by:
    TABLE "log_entries" CONSTRAINT "log_entries_field_id_fkey" FOREIGN KEY (field_id) REFERENCES log_fields(id)
    TABLE "task_log_entries" CONSTRAINT "task_log_entries_field_id_fkey" FOREIGN KEY (field_id) REFERENCES log_fields(id)

score 4 · Accepted Answer

查询大概可以简化为：

SELECT u.name AS user_name
     , p.name AS project_name
     , tl.created_on::date AS changeday
     , coalesce(sum(nullif(new_value, '')::numeric), 0)
     - coalesce(sum(nullif(old_value, '')::numeric), 0) AS hours
FROM   users             u
LEFT   JOIN (
        tasks            t 
   JOIN fixins           f  ON  f.id = t.fixin_id
   JOIN projects         p  ON  p.id = f.project_id
   JOIN task_log_entries tl ON  tl.task_id = t.id
                           AND  tl.field_id = 18
                           AND (tl.created_on IS NULL OR
                                tl.created_on >= '2013-09-08' AND
                                tl.created_on <  '2013-09-09') -- upper border!
       ) ON t.assignee_id = u.id
WHERE  EXISTS (SELECT 1 FROM tasks t1 WHERE t1.assignee_id = u.id)
GROUP  BY 1, 2, 3
ORDER  BY 1, 2, 3;

这将返回曾经有过任何任务的所有用户。
加上每个项目的数据和数据存在于指定日期范围内的日期task_log_entries。

要点

聚合函数sum()忽略NULL值。一旦您将计算重新转换为两个总和的差，就不再需要COALESCE() 每行：
```
 ,coalesce(sum(nullif(new_value, '')::numeric), 0) -
  coalesce(sum(nullif(old_value, '')::numeric), 0) AS hours
```
但是，如果选择的所有列都可能有NULL字符串或空字符串，则将总和包装COALESCE一次。
我正在使用更安全numeric的替代float方法来最大程度地减少舍入误差。
您尝试从usersand的连接中获得不同的值tasks是徒劳的，因为您再task一次向下连接。展平整个查询，使其更简单、更快速。
这些位置引用只是一个符号方便：
```
GROUP BY 1, 2, 3
ORDER BY 1, 2, 3
```
... 执行与原始查询相同的操作。
要从 a 中获取 a date，timestamp您可以简单地转换为date：
```
tl.created_on::date AS changeday
```
WHERE但是在子句或条件中使用原始值进行测试要好得多JOIN（如果可能，这里也有可能），因此 Postgres 可以在列上使用普通索引（如果可用）：
```
 AND (tl.created_on IS NULL OR
      tl.created_on >= '2013-09-08' AND
      tl.created_on <  '2013-09-09')  -- next day as excluded upper border
```
请注意，日期文字将转换为您当前时区timestamp的00:00日期。您需要选择第二天并将其排除为上边界。或者提供更明确的时间戳文字，例如. 更多关于排除上边界： '2013-09-22 0:0 +2':: timestamptz
- 计算 SQL 中的并发事件数
- 在 PostgreSQL 中查找重叠的日期范围
对于要求every user who has ever been assigned to a task添加WHERE子句：
```
WHERE EXISTS (SELECT 1 FROM tasks t1 WHERE t1.assignee_id = u.id)
```
最重要的是： ALEFT [OUTER] JOIN保留连接左侧的所有行。在右表上添加WHERE子句可以使这种效果无效。相反，将过滤器表达式移至子句。更多解释在这里： JOIN
- 使用 LEFT JOIN 查询不返回计数为 0 的行
括号可用于强制连接表的顺序。简单查询很少需要，但在这种情况下非常有用。我使用该功能加入 task, fixins,projects和task_log_entries之前将所有内容左加入users- 没有子查询。
表别名使编写复杂查询更容易。

score 1 · Accepted Answer

它不起作用，因为第一个查询是与任务内部连接的。同一个表用于执行外部联接（通过子查询但仍然如此），但第一个查询（任务用户）首先没有相关记录（缺少匹配项）。

尝试使用

....
FROM (
  SELECT DISTINCT
    users.id,
    users.name AS user_name
  FROM users    
) tasked_users
...

sql - 左外连接的作用类似于内连接

概括

示例查询

解释输出

表定义

2 回答 2

要点

Related

Reference