SQL Count items in each category 仅选择两个可用类别

Question

I have a table like so:

Year	Stage	Participant
2000	1	Peter
2000	1	John
2000	1	Daniel
2000	2	Peter
2000	2	John
2000	2	Steve
2000	3	Daniel
2000	3	John
2001	1	Peter
2001	1	John
2001	1	Daniel
2001	2	Peter
2001	2	Steve
2001	3	Daniel
2001	3	John
2001	4	Peter
2001	4	John
2001	4	Daniel

And what I need is to count the Participants in the first and last Stage and sort by year.

So far I got this

SELECT year, stage, Count(Participant) as Participants FROM table
WHERE Stage=1 or Stage=????
GROUP BY Stage
ORDER by Year

The problem is that each year has different amount of stages so I'm having problems on how to find and set as a parameter the LAST stage of each year

The output should look like this:

Year	Stage	Participants
2000	1	3
2000	3	2
2001	1	3
2001	4	3

Answer 1

This should work for both MySQL & SQL Server:

WITH CTE AS(
  SELECT Year,MIN(Stage) AS FirstStage, MAX(Stage) AS LastStage
  FROM Table1
  GROUP BY Year
)
SELECT T.Year,T.Stage,COUNT(Participant) AS Participants
FROM Table1 T
JOIN CTE ON T.Year = CTE.Year AND (T.Stage = CTE.FirstStage OR T.Stage = CTE.LastStage)
GROUP BY T.Year,T.Stage

Output:

Year	Stage	Participants
2000	1	3
2000	3	2
2001	1	3
2001	4	3

See this db<>fiddle

Answer 2

Using your existing query and just filling in "the blanks" you can use a correlated subquery

select year, stage, Count(*) as Participants 
from t
where Stage = 1
   or Stage = (select Max(stage) from t t2 where t2.year = t.year)
group by Stage, Year
order by year

This is likely to be the most performant but other variations are possible, such as using window functions:

select distinct year, stage, Participants
from (
  select Year, stage,
    First_Value(stage) over(partition by year order by stage) Fstage,
    First_Value(stage) over(partition by year order by stage desc) Lstage,
    Count(*) over(partition by year, stage) Participants
  from t
)t
where stage in (Fstage, Lstage)