If I assign a NVL constraint to a nullable column will it display an error or it will retrieve the records

I have a database in which some tables have to be joined to fetch the data. The problem is that in one table the column is nullable (nullable is set to yes or no constraint given for that column) and the select query is given as NVL(column name,0).

So will the command retrieve it normally or give an error?

Example:

Select  eid,ename ,nvl(sal,0),nvl(comm,0)
    from employee,sal
   where sal>1500
         AND
         comm=500

In the above query sal column is nullable but I am passing the query as nvl(sal,0) so will it retrieve the records or pass an error? If an error then how do you suggest I change the query?

I have already tried and getting neither the output nor any error in sql developer.

When tried in Plsql it gives PLS-00103: Encountered the symbol "OPEN" when expecting one of the following:

:= . ( @ % ;

Update:-

 cursor c_alerts is
select function_code               as function_code,
       nvl(latitude,0.0)           as latitude,
       nvl(longitude,0.0)          as longitude,
       nvl(gps_timestamp,0)        as gps_timestamp,
       nvl(violation_maxspeed, 0)  as violation_maxspeed,
       nvl(violation_start_time, 0)as violation_startspeed,
       nvl(violation_stop_time, 0) as violation_stopspeed,
       nvl(duration_sec, 0)        as duration_sec,
       cs_rec_insert_time          as rec_insert_time,
       nvl(location1, 'not found ')         as location1
  from t_vmu_alerts_hist
 where vmu_id = lv_vmu_id
   and gps_timestamp between lv_from_date and lv_to_date
union
select function_code               as function_code,
       nvl(latitude, 0.0)          as latitude,
       nvl(longitude, 0.0)         as longitude,
       gps                         as gps_timestamp,
       nvl(violation_maxspeed, 0)  as violation_maxspeed,
       nvl(violation_startspeed, 0)as violation_startspeed,
       nvl(violation_stopspeed, 0) as violation_stopspeed,
       nvl(duration_sec, 0)        as duration_sec,
       rec_insert_time             as rec_insert_time,
       loc_info                    as location1
  from p_error_vmu_alerts
 where vmu_id = lv_vmu_id
   and gps_timestamp between lv_from_date and lv_to_date
union
select function_code               as function_code,
       nvl(latitude, 0.0)          as latitude,
       nvl(longitude, 0.0)         as longitude,
       gps_timestamp               as gps_timestamp,
       nvl(violation_maxspeed, 0)  as violation_maxspeed,
       nvl(violation_startspeed, 0)as violation_startspeed,
       nvl(violation_stopspeed, 0) as violation_stopspeed,
       nvl(duration_sec, 0)        as duration_sec,
       rec_insert_time             as rec_insert_time,
       loc_info                    as location1
  from p_vmu_alerts
 where vmu_id = lv_vmu_id
   and gps_timestamp between lv_from_date and lv_to_date;

t_alert
    open c_alerts();
    FETCH c_alerts BULK COLLECT
      INTO t_alert;
    CLOSE c_alerts;
    LOG_ERROR := 13;