[SalesForce] GROUP BY Clause SOQL

Yes it's possible to group by custom fields in general, but not all field types are groupable.

The docs aren't particularly helpful from what I can see, and just tell you a field is groupable if it's describe field result has a groupable value of true.

IFIRC, you can group by pretty much anything, except DateTime, Long Text Areas, and Formulas, but unfortunately I can't seem to find any explicit documentation of this.

If you want to group by a formula field you can get by with by having creating a copy of the field and having workflow populate it, e.g. whenever your email domain field changes, do a field update that puts the value into the email domain (text) field. You can then use that to group.

I don't think you can run a query like that. But you can define a map of all the month numbers to zeros, then run your query and push the data into the map. Any missing months will not go into the map, so those values will remain at zero.

map<integer, integer> monthToCount = new map<integer, integer>{
  1 => 0,
  2 => 0,
  3 => 0,
  4 => 0,
  5 => 0,
  6 => 0,
  7 => 0,
  8 => 0,
  9 => 0,
  10 => 0,
  11 => 0,
  12 => 0
};
for (aggregateResult ar : [ SELECT CALENDAR_MONTH(Hire_Date__c) month_number,
                            COUNT(Id) num_employees FROM Employee__c
                            GROUP BY CALENDAR_MONTH(Hire_Date__c)])
{
   monthToCount.put( (integer) ar.get('month_number') , (integer) ar.get('num_employees'));
}