improve Query Builder to make more complex queries

For sub-queries, I believe Query Grouping already handles that, even in CI. That aside, I don't understand how this example would work. You're comparing a field with a partially generated query. How would it know what you're expecting to compare it to?

Grouping queries looks ugly. I want to suggest using an anonymous function.

PHP Code:

$builder->where(function(BaseBuilder $builder) {
   return $builder->where('a', 'a')
       ->orWhereNot(function(BaseBuilder $builder) {
            return $builder->where('b', 'b')->where('c', 'c');
       });
})->orWhereNot('d', 'd');

// SELECT * FROM `test` WHERE ( `a` = 'a' OR NOT ( `b` = 'b' AND `c` = 'c' ) ) OR `d` != 'd' 


Subqueries?

PHP Code:

$builder->where('key', function(BaseBuilder $builder) {
    return $builder->where('b', 'b')->select('key')->from('test_2');
});

// SELECT * FROM `test` WHERE key = (SELECT `key` FROM `test_2` WHERE `b` = 'b')

$builder->whereIn('key', function(BaseBuilder $builder) {
    return $builder->where('b', 'b')->select('key')->from('test_2');
});
// SELECT * FROM `test` WHERE `key` IN (SELECT `key` FROM `test_2` WHERE `b` = 'b') 


it's easy

You show submit a PR of the uploaded file, it would be easy to review.