Select when one of filter-column may not exists

Suppose a situation when it is not known if a filter column exists.

t:([]a:`s1`s2`s3; c:1 2 3);
select c from t where null t[`a]
select c from t where null t[`b]
'length
(where null t[`a])~where null t[`b]
1b

It column a exists, then the select is ok.

But when I use a filter with column b (not exists) then I get an error.

1. Why is it happening? - I've checked both of where results - they are the same
2. How to resolve this situation?

In QSQL 'where' needs to act on a list of booleans the same length of the table or of count 1. The count of your example is zero

q)t[`b]
`symbol$()
q)count t`b
0

Some examples:

q)select from t where 0b
a c
---
q)select from t where 00b
'length
  [0]  select from t where 00b
       ^
q)select from t where 000b
a c
---
q)select from t where 0000b
'length
  [0]  select from t where 0000b
       ^
q)select from t where 0#0b
'length
  [0]  select from t where 0#0b

Update: Because you mention the results of 'where', where is wrapped in the QSQL format as opposed to acting independently then the result being applied, so you see the below difference

q)where 00b
`long$()
q)select from t where 00b
'length
  [0]  select from t where 00b
       ^
q)select from t `long$()
a c
---