Question

我有两个表，父表（id，name，child_id）和孩子（id，姓名，号码） - 并非所有父母都可能有孩子而并非所有孩子都有父母。我需要一个查询来选择子表中所有记录的总和，并且还只选择那些具有父项和那些不具有的记录的总和 - 由父表child_id列确定。怎么办呢？

select
  sum(c.number) AS sum AS a,
  sum(all_child_records_that_have_a_parent) AS b,
  sum(all_child_records_that_do not have a parent) AS c /*do not use a-b if possible*/
from
  child c

＆＃34; all_child_records_that_have_a_parent＆＃34;是我无法弄清楚的：）

Answer 1

all_child_records_that_do没有父母：

SELECT *
  FROM child
 WHERE id NOT IN (SELECT child_id FROM parent)

Answer 2

您可以从父表中选择不同的子ID，并将它们外部连接到子表。然后检查是否为NULL。

select
  sum(c.number) AS sum_all_c,
  sum(case when x.child_id is not null then c.number end) AS sum_c_with_parent,
  sum(case when x.child_id is null then c.number end) AS sum_c_without_parent
from child c
left outer join (select distinct child_id from parent) x on x.child_id = c.id;

用于选择另一个表中引用的所有记录的总和的查询

2 个答案: