mysql - 加入时选择空值的Mysql

Question

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,
  PRIMARY KEY (`id`),
  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',null),
(3, 'dp3',1),
(4, 'dp4',2);

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

我得到的最接近的是以下查询。

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    |

由于某种我无法弄清楚的原因，我错过了最后一个元组。我想是因为这个加入 as tmp on tmp.department = d.id对于任何想要使用它和/或可以提供帮助的人，请按照 fiddle : Fiddle
我尝试了以下帖子，但没有运气。我现在卡住了。

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

how-to-join-tables-in-mysql-with-empty-table

Answer 1

您实际需要的是 2 个查询的完整外部联接。
问题是MySql(仍然)不支持这种连接，所以必须用左右连接和联合所有来模拟它:

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, d.father, p.sum_print
from d right join p
on p.department = d.id
where d.id is null

见 demo .
结果:

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