PostgreSQL：如果存在，则选择所有具有条件的行，否则忽略条件并选择最早的行

Question

我有一个返回 SETOF records 的 PostgreSQL 函数 find_all_records_with_params()。记录表有以下列：

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

我想选择 current_date - created 大于 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 created 日期的记录。

我尝试使用递归 cte（如下所示）来执行此操作，但它不允许 if/else 逻辑。我试图避免调用find_all_records_with_params() 函数两次，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 ;

我已使用联合查询来实现您所需的结果。如果您的过滤记录返回结果，则联合后查询不会产生任何结果，否则联合后查询会产生结果。