Select Minimum Date within WHERE Clause

Question

From a PostgreSQL perspective: I have a dataset which consists of indicator values, timestamps, and a flag to denote whether the indicator values pertain to the baseline or follow-up period.

I want to select all rows from the baseline period AND the first row from the follow-up period. I'm not able to use MIN(date) in a WHERE clause, and it's not that the follow-up period will always start on the same date in all cases.

Is there a way that I can do something like WHERE type = 'baseline' OR (type = 'fol' AND date <= MIN(date))?

From a Superset perspective: I'm using an Annotation Layer to highlight the portion of the indicator line that represents the baseline period. I then highlight the follow-up period with a different color. I need to make sure the highlighting is continuous (right now the line segment between baseline last date and the follow-up last date is not highlighted). So, I need to apply a filter to the chart underlying the baseline highlight Annotation Layer that returns data for the baseline period AND the first follow-up date.

Answer 1

There're several ways of doing this as I see:

1) Use UNION to have two separate queries

select <Query> where type = 'baseline'
UNION ALL
select <Query> where type = 'fol' order by date limit 1

2) Use the where clause you wrote, but calculate the date in subquery as in @Kushal Arya answer

3) Use window-function ROW_NUMBER

select
    <Query>,
    ROW_NUMBER() over (partition by type
                       order by date) as row_num
where
  type = 'baseline' OR
  (type = 'fol' AND row_num = 1)

I didn't run these queries