Let's say you only wanna select the first column. Cannot, it says 'NO_COLUMN: A'.
=query({O2:P4;O8:P9},"select A")
It is easy to select ALL columns.
=query({O2:P4;O8:P9},"select *")
Another option is to use the index of columns you want to select. The advantage here is that you can make complex references to other spreadsheets and data ranges that you would not be able to do easily using the "select" statement. Below is an example with simple references though. For example: if you need to select columns A and E (1st and 5th column) you would use the following formulae: ={index({A:X},0,1),index({A:X},0,5)}