PostgreSQL：如果存在條件選擇所有行，否則忽略條件並選擇最早的行

Question

我有一個PostgreSQL函數find_all_records_with_params()，它返回一個SETOF records。該記錄表有以下幾列：

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

我想選擇所有記錄，其中current_date - created大於90天。但是，如果此查詢不存在，我想選擇最早的created日期的1條記錄。

我試圖用遞歸cte（下面顯示）來做到這一點，但它不允許if/else邏輯。我試圖避免調用find_all_records_with_params()函數兩次，如果可能的話，我不希望創建臨時表。

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 ; 

我已經使用了聯合查詢，實現您想要的結果。如果您的filtered_records返回結果，則在聯合產生結果後，聯合將不會產生任何結果else查詢。