PostgreSQL - conditional ordering

问题

I have the following table:

key | date         | flag
--------------------------
1    now()           true
2    now() - 1 hour  true
3    now() + 1 hour  true
4    now()           false
5    now() - 1 hour  false
6    now() + 1 hour  false

I want the following sorting:

• First, all rows with flag = false. These rows must be sorted with date asc.
• Then, all other rows (flag = true). However, these rows must be sorted with date desc.

Is the following query correct?

(
    select *
    from test
    where flag = false
    order by date asc
)
union all
(
    select *
    from test
    where flag = true
    order by date desc
)

Is there a better way to do this? Will the union all keep the rows sorted, and therefore just concatenate the output of the two inner queries?

I don't know how to repeat the columns in the order by, based on a condition.

update

Fiddle can be found here: http://rextester.com/FFOSS79584

回答1:

conditional order can be performed with CASE, like here:

select *
    from test
order by 
    flag
  , case when flag then date end desc
  , case when not flag then date end asc

来源：https://stackoverflow.com/questions/43322046/postgresql-conditional-ordering