PostgreSQL : using a statement as case condition

Question

当我使用语句作为case 条件时，它总是返回false；

select * from table order by (case when (true) then id else 1/0 end)  desc -- works 
select * from table order by (case when (select true) then id else 1/0 end)  desc -- exception
select * from table order by (case when (1=1) then id else 1/0 end)  desc -- works
select * from table order by (case when (select 1=1) then id else 1/0 end)  desc -- exception

这种情况有什么问题？

Answer 1

CASE WHEN 期望根据 documentation 的条件得到一个 boolean 结果:

Each condition is an expression that returns a boolean result.

SELECT 语句返回一个关系(是的，单行有单列，boolean 类型和值 TRUE 但这仍然不是一个 boolean)。