select max() and 2nd highest

Yes but in a messy way, something along the lines of:-
select max(a),b

from mytable,(select max(a) maxa,b from mytable group by <img src='https://asprunner.com/forums/file.php?topicimage=1&fieldname=reply&id=29501&image=1&table=forumreplies' class='bbc_emoticon' alt='B)' /> z

where mytable.a<z.maxa and mytable.b=z.b

group by b

Yes but in a messy way, something along the lines of:-

Thanks a lot, it indeed appears to be what I need. Could you precise for the following situation:
I've got the groups of variables according to the indeterminate number of dates with indeterminate datediff (let say a1, a2, a2 for the dates d1, d2, d3...). I would like joining table with itself (alias) in order to have a table like
a1 d1 a2 d2 a3 d3 ...
I see no other way but to select variables a1 where d1 = min(d), a2 where d1<d2 and d2>d3, etc.
Still, I would like to limit the number of the dates to the realistic one, say 10 where d10 =max(d)

Just keep extending the above, but you'll get to the point where there are so many subselects that if you've a lot of data then you'll have an issue.
Depending on your database and the number that you have, you may be better selecting the keys then selecting top 10 x from mytable where y=nnn order by x desc

Just keep extending the above, but you'll get to the point where there are so many subselects that if you've a lot of data then you'll have an issue.

Not sure if I get you...
Sure I would like to select the keys (one by one) but the top 10 I will get only for all the table (limit, etc.)...
Lets see, I have parenttable with primary id= ID

child table on ID with primary autoincremental subId
every ID has a different nb of subId. I need to group those subId's (with belonging variables) horizontally for each ID.