how do I write a proper query in kdb this case?

Question

I would like to get all the groups that have a distinct price of 0 from my table, IE only if all prices are 0 in that group should it be returned.

My query & table look something like this.

tab:([]grp:`a`b`c`c`a`a`a;price:0 20 0 1 0 0 0)
select grp from tab where distinct price = 0

The output should only be `a since `a is the only group where all prices are 0.

Answer 1

Using an fby is one way to achieve the result here.

q)tab:([]grp:`a`b`c`c`a`a`a;price:0 20 0 1 0 0 0)
q)select from tab where 0=(max;abs price)fby grp
grp price
---------
a   0
a   0
a   0
a   0
q)distinct exec grp from tab where 0=(max;abs price)fby grp
,`a