Похоже, я нашел ответ: => означает, что равен
group by A,B => group by A,B
group by B,C => group by C => group by A,B,C
group by A,C => group by C => group by A,B,C
group by A => group by A
group by B => group by A,B
group by C => group by A,B,C
group by B,C => group by A,B,C