How to add results of two select commands in same query

Yes. It is possible :D

SELECT  SUM(totalHours) totalHoursFROM        (             select sum(hours) totalHours from resource            UNION ALL            select sum(hours) totalHours from projects-time        ) s

As a sidenote, the tablename projects-time must be delimited to avoid syntax error. Delimiter symbols vary on RDBMS you are using.

Something simple like this can be done using subqueries in the select clause:

select ((select sum(hours) from resource) +        (select sum(hours) from projects-time)       ) as totalHours

For such a simple query as this, such a subselect is reasonable.

In some databases, you might have to add from dual for the query to compile.

If you want to output each individually:

select (select sum(hours) from resource) as ResourceHours,       (select sum(hours) from projects-time) as ProjectHours

If you want both and the sum, a subquery is handy:

select ResourceHours, ProjectHours, (ResourceHours+ProjecctHours) as TotalHoursfrom (select (select sum(hours) from resource) as ResourceHours,             (select sum(hours) from projects-time) as ProjectHours     ) t

UNION ALL once, aggregate once:

SELECT sum(hours) AS total_hoursFROM   (   SELECT hours FROM resource   UNION ALL   SELECT hours FROM "projects-time" -- illegal name without quotes in most RDBMS   ) x