KDB/Q-sql Dynamic Grouping and con-canting columns in output - kdb

Question

I have a table where I have to perform group by on dynamic columns and perform aggregation, result will be column values concatenating group-by tables and aggregations on col supplied by users.
For example :
g1 g2 g3 g4 col1 col2
A D F H 10 20
A E G I 11 21
B D G J 12 22
B E F L 13 23
C D F M 14 24
C D G M 15 25
and if I need to perform group by g1,g2,g4 and avg aggregation on col1 output should be like this
filed val
Avg[A-D-H-col1] 10.0
Avg[A-E-I-col1] 11.0
Avg[B-D-J-col1] 12.0
Avg[B-E-L-col1] 13.0
Avg[C-D-M-col1] 14.5
I am able to perform this if my group by columns are fixed using q-sql
t:([]g1:`A`A`B`B`C`C;g2:`D`E`D`E`D`D;g3:`F`G`G`F`F`G;g4:`H`I`J`L`M`M;col1:10 11 12 13 14 15;col2:20 21 22 23 24 25)
select filed:first ("Avg[",/:(({"-" sv x} each string (g1,'g2,'g4)),\:"-col1]")),val: avg col1 by g1,g2,g4 from t
I want to use functional query for the same , means I want a function which take list of group by columns, aggregation to perform and col name andtable name as input and output like above query. I can perform group by easily using dynamic columns but not able to con-cat in fields. function signature will be something like this
fun{[glist; agg; col,t] .. ;... }[g1g2g4;avg;col1,t]
Please help me to make above query as dynamic.

Answer 1

You may try following function:
specialGroup: {[glist;agg;col;table]
res: ?[table;();{x!x}glist; enlist[`val]!enlist(agg;col)];
aggname: string agg;
aggname: upper[1#aggname], 1_aggname;
res: ![res;();0b;enlist[`filed]!enlist({(y,"["),/:("-"sv/:string flip x),\:"]"};enlist,glist,enlist[enlist col];aggname)];
res
};
specialGroup[`g1`g2`g4;avg;`col1;t]
specialGroup aggregates values into val column first. And populates filed column after grouping. This helps to avoid generating filed duplicates and selecting first of them.

Answer 2

If you modify Anton's code to this it will change the output dynamically
specialGroup: {[glist;agg;col;table]
res: ?[table;();{x!x}glist; enlist[`val]!enlist(agg;col)];
res: ![res;();0b;enlist[`filed]!enlist({(#[string[y];0;upper],"["),/:("-"sv/:string flip x),\:"]"}[;agg];enlist,glist,enlist[enlist col])];
res
};
As the part of the code that made that string was inside another function you need to pass the agg parameter to the inner function.