We can group by several columns in kdb+:
q)t:([]a:1 1 3;b:1 1 4;c:7 8 9);
q)select sum c by grp:([]a;b) from t
grp | c
--------| --
`a`b!1 1| 15
`a`b!3 4| 9
q)gcols:`a`b
But how to do the same with a functional form (how to build a correct parse tree) if desired columns to group by are in a variable gcols?
The more conventional group by would be:
q)select sum c by a,b from t
a b| c
---| --
1 1| 15
3 4| 9
for which the functional form would be
q)?[t;();{x!x}gcols;(1#`c)!enlist(sum;`c)]
a b| c
---| --
1 1| 15
3 4| 9
The functional form for your style of grouping would be:
q)?[t;();(1#`grp)!enlist(flip;(!;enlist gcols;enlist,gcols));(1#`c)!enlist(sum;`c)]
grp | c
--------| --
`a`b!1 1| 15
`a`b!3 4| 9