按值对行进行分组和计数，直到更改为止

I have a table where messages are stored as they happen. Usually there is a message 'A' and sometimes the A's are separated by a single message 'B'. Now I want to group the values so I'm able to analyze them, for example finding longest 'A'-streak or distribution of 'A'-streaks.I have a table where messages are stored as the