Question

假设我有此查询以供查看：

select a.name as Author,b.name as Book
from a
left join b on a.id = b.a_id

并产生以下输出：

Brandon Sanderson Mistborn
Brandon Sanderson Way of Kings
Brandon Sanderson Steelheart
Patrick Rothfuss  The Wise Man's Fear
Patrick Rothfuss  The Name of the Wind

到目前为止一切都很好，但我想将重复项隐藏在左行：

Brandon Sanderson Mistborn
                  Way of Kings
                  Steelheart
Patrick Rothfuss  The Wise Man's Fear
                  The Name of the Wind

这使IMO的阅读更加容易，这就是我要用于此视图的格式。

我该如何实现？

Answer 1

好吧，这可以通过window function来实现，如果您现在使用的数据库支持此功能，例如PostgreSQL或高版本（8.0+）的MySQL，则可以编写如下的SQL：

select * from books;
      author       |         book         
-------------------+----------------------
 Brandon Sanderson | Mistborn
 Brandon Sanderson | Way of Kings
 Brandon Sanderson | Steelheart
 Patrick Rothfuss  | The Wise Man’s Fear
 Patrick Rothfuss  | The Name of the Wind
(5 rows)

select
    case when row_number() over(partition by author order by book) > 1 then null else author end as author,
    book
from
    books;
      author       |         book         
-------------------+----------------------
 Brandon Sanderson | Mistborn
                   | Steelheart
                   | Way of Kings
 Patrick Rothfuss  | The Name of the Wind
                   | The Wise Man's Fear
(5 rows)

with tmp as (
select
    author,
    book,
    row_number() over(partition by author order by book) as sort
from
    books
)
select
    case when sort>1 then null else author end as th_author,
    book
from
    tmp
order by
    author,sort;
     th_author     |         book         
-------------------+----------------------
 Brandon Sanderson | Mistborn
                   | Steelheart
                   | Way of Kings
 Patrick Rothfuss  | The Name of the Wind
                   | The Wise Man‘s Fear

或者数据库不支持window function，您可以编写如下内容：

with named_book as (
select
    author,
    min(book) as book
from
    books
group by
    author
)
select
    b.author,
    a.book
from
    books a
left join
    named_book b on a.book = b.book
order by
    a.author,a.book;
      author       |         book         
-------------------+----------------------
 Brandon Sanderson | Mistborn
                   | Steelheart
                   | Way of Kings
 Patrick Rothfuss  | The Name of the Wind
                   | The Wise Man's Fear
(5 rows)

Answer 2

这不是您通常在SQL中要做的事情。表格具有行和列，并且-通常-一行中的值sin应该独立于其他行。

您可以使用row_number()进行此操作，请注意排序标准：

select (case when 1 = row_number() over (partition by a.id order by b.name)
             then a.name
        end) as Author,
       b.name as Book
from a left join
     b
     on a.id = b.a_id
order by a.id, b.name;

外部order by键与partition by和order by子句中的键相匹配非常重要。

您可能会发现为每个作者创建一行更为有用。语法因数据库而异，但我认为标准功能是listagg()：

select a.name as Author,
       listagg(b.name, '; ') within group (order by b.name) as Book
from a left join
     b
     on a.id = b.a_id
group by a.name;

隐藏左连接列上除第一行以外的所有数据

2 个答案: