PostgreSQL：选择具有条件的所有行（如果存在），否则忽略条件并选择最早的行

Question

我有一个 PostgreSQL 函数find_all_records_with_params()，它返回一个SETOF records. 记录表具有以下列：

table records(
id integer,
type integer,
created date,
content text
);

我想选择current_date - created超过 90 天的所有记录。但是，如果此查询不存在，我想选择 1 条最早日期的记录created。

我尝试使用递归 cte（如下所示）来执行此操作，但它不允许使用 if/else 逻辑。我试图避免调用该find_all_records_with_params()函数两次，并且如果可能的话，我不希望创建临时表。

with most_records as (select * from find_all_records_with_params()),
filtered_records as (select * from most_records where current_date - created > 90)
if exists(select * from filtered_records) then
select * from filtered_records;
else
select * from most_records order by created asc limit 1;
end if;

Answer 1

with most_records as (select * from find_all_records_with_params()),
filtered_records as (select * from most_records where current_date - created > 90)
select * from filtered_records
union 
select * from 
(select * from most_records 
where not exists (select * from filtered_records limit 1) 
order by created asc limit 1 ) a ;

我已经使用联合查询来达到您所需的结果。如果您的filtered_records返回结果，则联合后的查询将不会产生任何结果，否则联合后的查询将产生结果。