选择连接时值为空的Mysql

如何解决选择连接时值为空的Mysql

我有以下MysqL表（创建和插入）：

CREATE TABLE IF NOT EXISTS `department` (
  `id` INT NOT NULL,`name` VARCHAR(45) NOT NULL,`father` INT NULL,PRIMARY KEY (`id`),INDEX `fk_department_department_idx` (`father` ASC) VISIBLE,CONSTRAINT `fk_department_department`
    FOREIGN KEY (`father`)
    REFERENCES `department` (`id`)
    ON DELETE NO ACTION
    ON UPDATE NO ACTION)
ENGINE = InnoDB;

CREATE TABLE IF NOT EXISTS `print` (
  `id` INT NOT NULL,`page` INT NOT NULL,`copy` INT NOT NULL,`date` DATE NOT NULL,`department` INT NULL,INDEX `fk_print_department1_idx` (`department` ASC) VISIBLE,CONSTRAINT `fk_print_department1`
    FOREIGN KEY (`department`)
    REFERENCES `department` (`id`)
    ON DELETE NO ACTION
    ON UPDATE NO ACTION)
ENGINE = InnoDB;

insert into department (id,name,father)
values
(1,'dp1',null),(2,'dp2',(3,'dp3',1),(4,'dp4',2);

insert into print (id,page,copy,date,department)
values
(1,2,3,'2020-1-11',1,6,'2020-1-12',4),4,5,'2020-1-13',(5,'2020-1-15',(6,(7,(8,'2020-1-16',4);

最近的查询是以下查询。

select
    d.name as department,f.name as father,sum_print as sum
from
    department d
left join department f on f.id = d.father
left join
(
  select 
    department,sum(page*copy) as sum_print
  from print
  where date between CAST('2020-1-13' AS DATE) 
                        AND    CAST('2020-1-15' AS DATE)
  group by department
) as tmp on tmp.department = d.id;

我得到的结果是。

|  department  |  father  |  sum  |
|     dp1      |   null   |  24   |
|     dp2      |   null   | null  |
|     dp3      |   dp1    | null  |
|     dp4      |   dp2    | null  |

我需要的是以下内容。

|  department  |  father  |  sum  |
|     dp1      |   null   |  24   |
|     dp2      |   null   | null  |
|     dp3      |   dp1    | null  |
|     dp4      |   dp2    | null  |
|     null     |   null   | 35    |

由于某种原因，我无法弄清楚，我错过了最后一个元组。我认为是因为加入了as tmp on tmp.department = d.id

在所有想要使用它并且/或者可以帮助的人中关注小提琴：here

我尝试了以下帖子，但没有运气。我现在卡住了。

• Fiddle
• mysql-select-join-with-empty-row-returns
• mysql-inner-join-with-possible-empty-fields
• mysql-how-to-return-rows-even-if-join-data-is-empty

解决方法

您真正需要的是2个查询的完全外部联接。
问题在于MySql（仍然）不支持这种联接，因此必须使用左右联接模拟所有联接：

with 
  d as (
    select d.id,d.name as department,f.name as father
    from department d left join department f 
    on f.id = d.father  
  ),p as (
    select department,sum(page*copy) as sum_print
    from print
    where date between CAST('2020-1-13' AS DATE) AND CAST('2020-1-15' AS DATE)
    group by department  
  )
select d.department,d.father,p.sum_print
from d left join p
on p.department = d.id
union all
select d.department,p.sum_print
from d right join p
on p.department = d.id
where d.id is null

请参见demo。
结果：

> department | father | sum_print
> :--------- | :----- | --------:
> dp1        | null   |        24
> dp2        | null   |      null
> dp3        | dp1    |      null
> dp4        | dp2    |      null
> null       | null   |        35

,

这将为您提供所需的外部连接结果

uid

https://www.db-fiddle.com/f/xvfhaQcFQMS8BF1W9iYcn2/1