Sub queries

[gwillz]

All those fancy things we dreamed about!

This is subquery nested in the FRON statement like SELECT * FROM (...) AS subQuery.

$subQuery = new PdbQuery($pdb);

$query = (new PdbQuery($pdb))
    ->from($subQuery, 'foo');

$subQuery->from('bar')
    ->where(['name' => 'test'])
    ->select('id, category');

Some of the key magic here is that the subQuery isn't built or validated until the parent query is built. Meaning you're free to modify that object after attaching it to the parent.

This is a subquery using the WITH statement, which is somewhat like a temporary/inline VIEW.

$subQuery = $pdb->find('foo')
    ->select('id as _id')
    ->where(['name' => 'test']);

$query = $pdb->find('foo')
   ->innerJoin('sub', '~foo.id = sub._id')
   ->with($subQuery, 'sub');

Union also exists now, but that's pretty boring.

Having said all that. This PR description should go straight into some docs.

The test cases also need some real executed queries. Not just generated SQL.