如何进行不重复父值的连接

Question

我正在尝试制作一个不重复父值的 SQL。

查询：

select 
  p.name as parent_name,
  c.name as children_name
from 
  parent p left outer join children c using(parent_id);

结果是：

PARENT_NAME | CHILDREN_NAME
Parent 1    | Children 1
Parent 1    | Children 2
Parent 1    | Children 3
Parent 1    | Children 4
Parent 2    | Children 1
Parent 2    | Children 2
Parent 2    | Children 3
Parent 2    | Children 4

但是，我的期望是（就像向下钻取）：

PARENT_NAME | CHILDREN_NAME
Parent 1    | Children 1
<NULL>      | Children 2
<NULL>      | Children 3
<NULL>      | Children 4
Parent 2    | Children 1
<NULL>      | Children 2
<NULL>      | Children 3
<NULL>      | Children 4

SQL摆弄例子是：http://sqlfiddle.com/#!7/2e8f4/1

Answer 1

您可以使用 row_number() 和 case 表达式：

select 
    case when row_number() over(partition by p.parent_id order by c.id) = 1 then p.name end as parent_name,
    c.name as children_name
from parent p 
left outer join children c using(parent_id)
order by p.parent_id, c.id

如果您的 SQLite 版本不支持 window 函数，一种替代方法是相关子查询：

select 
    case when not exists (select 1 from children c1 where c1.parent_id = c.parent_id and c1.id < c.id) then p.name end as parent_name,
    c.name as children_name
from parent p 
left outer join children c using(parent_id)
order by p.parent_id, c.id

如何进行不重复父值的连接

How to make a join that not repeat parent values

sql

sqlite

case

sql-order-by

window-functions