Output null values for missing dates
If you're using SQLite, then while Peewee supports RIGHT OUTER, SQLite itself does not. Can you simply switch the order of your joins and use a left outer?
 You may need to add clauses to your where or join as well that allow for the date to be NULL. But, basically this is a SQL question not a peewee bug.

Hi,

I've tried and I've failed.

I have a table like so 

``` python
class ShortUsage(BaseModel):
    projid = ForeignKeyField(Project, to_field="projid")
    folder = CharField(index=True)
    user = ForeignKeyField(User, to_field="username")
    # Support only dates. Don't want sub-day timescales, makes it harder to join
    # data and snapshots are only taken daily
    scandate = DateField(index=True)
    inodes = FloatField()
    size = FloatField()
    class Meta:
        # Combination of projid+folder+user+date must be unique
        indexes = ( (('projid','folder','user','scandate'), True), )
```

For each projid and user I want to output the sum of size, which I can do no problem. Not all users have entries for all dates in a given range. I want to force the output of a value (zero?) for dates where they have no data. Basically, I need to output the sum of size for a superset of all dates within a given range.

I have succeeded in making a query which produces the dates I need for a given period:

``` python
SUalias = ShortUsage.alias()
q1 = SUalias.select(fn.Distinct(SUalias.scandate).alias('alldate')).where(SUalias.projid == projid & SUalias.scandate >= startdate & SUalias.scandate <= enddate).alias('q1')
```

but when I try and join that to my existing table

``` python
    for date, diskusage in ShortUsage.select(ShortUsage.scandate,fn.SUM(ShortUsage.size))\
        .where((ShortUsage.user == user) & (ShortUsage.projid == projid))\
        .join(q1, JOIN.LEFT_OUTER, on=(q1.c.alldate == ShortUsage.scandate))\
        .group_by(ShortUsage.scandate)\
        .order_by(ShortUsage.scandate).tuples():
        dates.append(date)
        usage.append(diskusage/1000.)
```

It does not report values for all the dates in q1. Is this because it is a LEFT_OUTER join? When I try a full join or RIGHT_OUTER I get this:

```
peewee.OperationalError: RIGHT and FULL OUTER JOINs are not currently supported
```

I am using v2.6.3, and the docs seem to suggest full joins are possible 

> By default peewee will use an INNER join, but you can use LEFT OUTER or FULL joins as well

Any assistance would be greatly appreciated.
