Question

我有这个非常简单的表

create table foo (id int);
create table bar (id int, name text);

insert into foo(id) values (1), (2);
insert into bar(id, name) values (1, 'a'), (1, 'b'), (2, 'c'), (2, 'd'), (2, 'e');

我想为每个bar.name连接foo，但限制bar.name的数量。

例如，要限制为2，应返回

id | names
1  | a,b
2  | c,d

没有限制，我知道我可以这样写

select id, group_concat(name) as names from foo natural join bar group by id;

这给了我

id | names
1  | a,b
2  | c,d,e

但是我不知道如何限制name到group_concat的范围。

我还如何订购name过去的group_concat？

Answer 1

在SQLite的最新版本中，您可以使用窗口函数：

select f.id, group_concat(b.name) as names
from foo join
     (select b.*,
             row_number() over (partition by b.id order by b.id) as seqnum
      from bar b
     ) b
     on b.id = f.id
where seqnum <= 2
group by f.id;

如果只需要两个值，也许min()和max()会满足您的要求：

select f.id, 
       (case when min(b.name) = max(b.name)
             then min(b.name)
             else min(b.name) || ',' || max(b.name)
        end) as names
from foo join
     bar b
     on b.id = f.id
group by f.id;

正如您所说的那样，join不是必需的：

select b.id, group_concat(b.name) as names
from (select b.*,
             row_number() over (partition by b.id order by b.id) as seqnum
      from bar b
     ) b
where seqnum <= 2
group by b.id;

Answer 2

使用一个子查询，该子查询可以从另一个有限的子查询中获取连接。

SELECT f.id,
       (SELECT group_concat(x.name)
               FROM (SELECT b.name
                            FROM bar b
                            WHERE b.id = f.id
                            ORDER BY b.name
                            LIMIT 2) x) names
      FROM foo f;

db<>fiddle

如何在sqlite中限制group_concat子组？

2 个答案: