Using user-defined variable with ORDER BY bad result set

I have a t1 table with records in some same and different 'f2' column values.
I would request records where the 'f2' column value not equal previous row's 'f2' column value.
If I use ORDER BY the result set is bad. All record present in the result set.
If I not use ORDER BY the result set is correct. Same query without ORDER BY. The result set is contains one row. (This is the correct result set.)

What is the correct SQL query for this request (without LIMIT expression)?