KDB Apply where phrase only if column exists - kdb

Question

I'm looking for a way to write functional select in KDB such that the where phrases is only apply if the column exists (on order to avoid error). If the column doesn't exist, it defaults to true.
I tried this but it didn't work
enlist(|;enlist(in;`colname;key flip table);enlist(in;`colname;filteredValues[`colname]));
I tried to write a simple boolean expression and use parse to get my functional form
(table[`colname] in values)|(not `colname in key flip table)
But kdb doesn't have short circuit so the left-hand expression is still evaluated despite the right-hand expression evaluating to true. This caused a weird output boolean$() which is a list of booleans all evaluating to false 0b
Any help is appreciated. Thanks!
EDIT 1: I have to join a series of condition with parameter specified in the dictionary filters
cond,:(,/) {[l;k] enlist(in;k;enlist l[k])}[filters]'[a:(key filters)]
Then I pass this cond on and it gets executed on a few different selects on different tables. How can I make sure that whatever conditional expression I put in place of enlist(in;k;enlist l[k] will only get evaluated as the select statement gets executed.

Answer 1

You can use the if-else conditional $ here to do what you want
For example:
q)$[`bid in cols`quotes;enlist (>;`bid;35);()]
> `bid 35
q)$[`bad in cols`quotes;enlist (>;`bad;35);()]
Note that in the second example, the return is an empty list, as this column isn't in quotes table
So you can put this into the functional select like so:
?[`quotes;$[`bid in cols`quotes;enlist (>;`bid;35);()];0b;()]
and the where clause will be applied the the column is present, otherwise no where clause will be applied:
q)count ?[`quotes;$[`bid in cols`quotes;enlist (>;`bid;35);()];0b;()]
541 //where clause applied, table filtered
q)count ?[`quotes;$[`bad in cols`quotes;enlist (>;`bad;35);()];0b;()]
1000 //where clause not applied, full table returned
Hope this helps
Jonathon
AquaQ Analytics
EDIT: If I'm understanding your updated question correctly, you might be able to do something a like the following. Firstly, let's define an example "filters" dictionary:
q)filters:`a`b`c!(1 2 3;"abc";`d`e`f)
q)filters
a| 1 2 3
b| a b c
c| d e f
So here we are assuming a few different columns of different types, for illustration purposes. You can build up your list of where clauses like so:
q)(in),'flip (key filters;value filters)
in `a 1 2 3
in `b "abc"
in `c `d`e`f
(this is equivalent to the code you had to generate cond, but it's a little neater & more efficient - you also have the values enlisted, which isn't necessary)
You could then use a vector conditional to generate your list of where clauses to apply to a given table e.g.
q)t:([] a:1 2 3 4 5 6;b:"adcghf")
q)?[key[filters] in cols[t];(in),'flip (key filters;value filters);count[filters]#()]
(in;`a;,1 2 3)
(in;`b;,"abc")
()
As you can see, in this example the table "t" has columns a and b, but not c. So using the vector conditional, you get the where clauses for a and b but not c.
Finally to actually apply this list of output where clauses to the table, you can make use of an over to apply each in turn:
q)l:?[key[filters] in cols[t];(in),'flip (key filters;value filters);count[filters]#()]
q){?[x;$[y~();y;enlist y];0b;()]}/[t;l]
a b
---
1 a
3 c
One thing to note here is that in the where clause of the functional select we need to check if y is an empty list - this is so we can enlist it if it is not an empty list
Hope this helps