PostgreSQL中不带if-else语句的条件选择

您可以添加 EXISTS 进入 WHERE 条款

select * 
from my_table 
where (EXISTS (select 1 from my_table where for_country='Germany') and for_country='Germany') OR
      (NOT EXISTS (select 1 from my_table where for_country='Germany') and for_country='All')

DBFiddle DEMO

一个可能更好的解决方案是 存在 和 CROSS JOIN 避免同一子查询的重复调用

select my_table.* 
from my_table 
cross join (
  select exists(select 1 from my_table where for_country='Germany') exst
) t
where (t.exst and for_country='Germany') OR
      (not t.exst and for_country='All')

DBFiddle DEMO

尝试以下查询:

select m1.name from
   (
    select m1.*, case when m1.for_region='Germany' then 1  else 0  end  as cnt from tableA m1 ) m1
     inner join 
    (
    select max(Cnt) as Cnt from
    (
     select t1.*, case when for_region='Germany' then 1  else 0  end  as Cnt 
      from tableA t1
     ) as t2

     )as n 
     on m1.cnt=n.Cnt

select * from my_table 
where 
((SELECT DISTINCT 1 FROM my_table where for_country='Germany') = 1 AND for_country='Germany')
OR for_country='All'

我会用 UNION 具有 NOT EXISTS :

SELECT * 
FROM my_table 
WHERE for_country = 'Germany'
UNION ALL
SELECT *
FROM my_table
WHERE for_country = 'All' AND
      NOT EXISTS (SELECT 1 FROM my_table WHERE for_country = 'Germany');

我会这样写:

select t.*
from my_table t.
where (for_country = 'Germany') or
      (not exists (select 1 from my_table where for_country = 'Germany') and
       for_country = 'All'
      );