Question

Given:

timestamp |  col1  |   col2
============================
xx            abc      5
yy            abc      4
zz            def      7
rr            def      6


SELECT timestamp,col1,min(col2) 
FROM table 
GROUP BY col1 
ORDER BY min(col2) ASC

returns:

xx       abc     4
zz       def     6

timestamp seems to be messed up, so I am sure I am not using group by the way it is supposed to. How do I get:

yy     abc      4
rr     def      6

Answer 1

For guaranteed behaviour you must either have the timestamp field in an aggregate expression or in the GROUP BY clause. If you don’t (as in your example), the returned value is indeterminent (and effectively almost randomly chosen).

Instead, you can do this in two steps, and so never have any field ‘randonly’ chosen…

SELECT
  *
FROM
  yourTable
INNER JOIN
  (SELECT col1, MAX(col2) as max_col2 FROM yourTable GROUP BY col1) AS lookup
    ON  yourtable.col1 = lookup.col1
    AND yourtable.col2 = lookup.col2