succesive rows query

[Oguz Yarimtepe]

Hi, 

At my table i have fields a, b  and lets say i want to calculate the
number of entries at the table which has the mentioned propery: if there
are consecutive rows with a=x, b=y and the next line a=y and b=x but the
third line should be a != x and b != y, is it possible with an sql query
on mysql and where should i check to learn the how to?

Thanx.