What is the best way to convert an int or null to boolean value in an SQL query?

To my knowledge (correct me if I'm wrong), there is no concept of literal boolean values in SQL. You can have expressions evaluating to boolean values, but you cannot output them.

This said, you can use CASE WHEN to produce a value you can use in a comparison:

SELECT 
    CASE WHEN ValueColumn IS NULL THEN 'FALSE' ELSE 'TRUE' END BooleanOutput 
FROM 
    table 

No need to use case... when:

select (column_name is not null) as result from table_name;

Returns 1 for all fields not NULL and 0 for all fields that are NULL, which is as close as you can get to booleans in SQL.

SELECT 
    CASE
        WHEN thevalue IS NULL THEN 0
        ELSE 1
    END AS newVal
FROM .... (rest of select)

I think it goes something like this

Actually, the ISNULL, may need to be WHEN thevalue IS NULL THEN 0

Assuming you want 0,1 value as a return, and that we are talking about integer I would use the logic specified by Torbjörn and wrap it in the function

create function dbo.isNotNull(@a int)
returns bit
as
begin
return isnull(@a-@a+1,0)
end

so then you can use it whenever you need by simply calling

select dbo.isNotNull(myIntColumn) from myTable

The answer provided by Tomalak is more universal though as it would work with any data type