Postgres选择'至少'项目

Question

我想选择评论发布，老年人然后特别commentId，但我想要有至少反正5结果。

所以，如果有少于5条评论是sql：SELECT * FROM comments WHERE id >= :comment_id，我必须再作一个选择SELECT * FROM comments LIMIT 5。

是否有可能在一个请求中获得相同的逻辑？

Answer 1

with c as (
    select count(*) as c 
    from comments 
    where id >= :comment_id 
) 
select * 
from comments 
where id >= :comment_id 
union all 
(
    select * 
    from comments 
    where id < :comment_id 
    order by id desc 
    limit greatest(5 - (select c from c), 0) 
) 
; 

Answer 2

尝试：

WITH x AS { 
    SELECT * FROM comments WHERE id >= :comment_id 
), 
y AS (
    SELECT * FROM comments 
    LIMIT 5 
) 
SELECT * FROM x 
WHERE 5 <= (SELECT count(*) FROM x) 
UNION ALL 
SELECT * FROM y 
WHERE 5 > (SELECT count(*) FROM x)