SQL group by: select value where another column has its min/max

Question

I want to group by one column, get both min and max of a second column, and (this is the tricky part!) get the value from a third column where the second column has its min value in the group.

Example:

MyTable:

ID     TS     GRP
==================
 1     20      A
 2     20      B
 3     10      A
 4     30      A
 5     10      B
 6     40      A

Desired result (ID should be the value from the record where TS has its minimum):

ID    MIN_TS   MAX_TS   GRP
============================
 3      10       40      A
 5      10       20      B

In general, the grouping query is very easy:

SELECT <???> AS ID, MIN(TS) AS MIN_TS, MAX(TS) AS MAX_TS, GRP
FROM MyTable
GROUP BY GRP

But what about the ID part? It doesn't work this way with grouping, right? But why? And what's the best workaround?

Answer 1

Do the aggregation in a subquery, then look up the ID for each group in another subquery:

SELECT
  (SELECT TOP(1) id FROM MyTable WHERE grp = agg.grp ORDER BY ts DESC) AS id,
  min_ts, max_ts, grp
FROM (SELECT min(ts) AS min_ts, max(ts) AS max_ts, grp
      FROM MyTable
      GROUP BY grp) agg

Or use window functions:

SELECT id, min_ts, max_ts, grp
FROM (SELECT 
        id,
        min(ts) OVER (PARTITION BY grp) min_ts,
        max(ts) OVER (PARTITION BY grp) max_ts,
        grp,
        row_number OVER (PARTITION BY grp ORDER BY ts) rn
      FROM MyTable)
WHERE rn = 1;

This query uses window functions to calculate min_ts and max_ts for each group, and then filters to only include the first row for each group (ordered by ts).