The current implementation of the database query where method is limited by an unfortunate restriction in that the first use of the glue parameter fixes the glue for all subsequent uses. For example,

$query->where(array('foo = 1', 'bar = 2'));
// WHERE foo = 1 AND bar = 2

$query->where(array('foo = 1', 'bar = 2'), 'OR');
// WHERE foo = 1 OR bar = 2

$query
->where(array('foo = 1', 'bar = 2'), 'AND')
->where(array('baz = 3', 'goo = 4'), 'OR');
// WHERE foo = 1 AND bar = 2 AND baz = 3 AND goo = 4
// Note that the OR glue is completely ignored in the second where statement.

This behaviour is inconvenient but can't be changed in Joomla 3.x because it would break backwards compatibility. Consequently, this PR offers the following additional syntax to support more complex WHERE clauses:

orWhere($conditions, $glue = 'AND')

$query
->where(array('foo = 1', 'bar = 2'))
->orWhere(array('baz = 3', 'goo = 4'));
// WHERE (foo = 1 AND bar = 2) OR (baz = 3 AND goo = 4)

$query
->where(array('foo = 1', 'bar = 2'))
->orWhere(array('baz = 3', 'goo = 4'), 'XOR');
// WHERE (foo = 1 AND bar = 2) OR (baz = 3 XOR goo = 4)

andWhere($conditions, $glue = 'OR')

$query
->where(array('foo = 1', 'bar = 2'))
->andWhere(array('baz = 3', 'goo = 4'));
// WHERE (foo = 1 AND bar = 2) AND (baz = 3 OR goo = 4)

$query
->where(array('foo = 1', 'bar = 2'))
->andWhere(array('baz = 3', 'goo = 4'), 'XOR');
// WHERE (foo = 1 AND bar = 2) AND (baz = 3 XOR goo = 4)

extendWhere($outerGlue, $conditions, $innerGlue = 'AND')

$query
->where(array('foo = 1', 'bar = 2'))
->extendWhere('XOR', array('baz = 3', 'goo = 4'));
// WHERE (foo = 1 AND bar = 2) XOR (baz = 3 AND goo = 4)

$query
->where(array('foo = 1', 'bar = 2'))
->extendWhere('XOR', array('baz = 3', 'goo = 4'), 'OR');
// WHERE (foo = 1 AND bar = 2) XOR (baz = 3 OR goo = 4)

The extendWhere method allows additional logical operators to be used beyond the commonly used AND and OR operators. For example, you can use XOR, AND NOT and OR NOT.

Backwards compatibility
There are no backwards compatibility issues. Existing code is unaffected because only new syntax is being added.

Testing
The code is accompanied by unit tests for the new methods and an extension to the where method test which verifies the original behaviour.