How many times does the word "riu" appears in the biwords pair?
riu refers to word = 'riu'; SUM(w1st where wid is word = 'riu' AND w2nd where wid is word = 'riu')
SELECT COUNT(T1.wid) FROM words AS T1 INNER JOIN biwords AS T2 ON T1.wid = T2.w1st INNER JOIN words AS T3 ON T3.wid = T2.w2nd WHERE T1.word = 'riu'