Calculate the percentage of times that the same word appears in a pair.
Pair is a relationship of two words: w1st and w2nd, where w1st is word id of the first word and w2nd is a word id of the second word; the same word appears in a pair refers to w1st = w2nd; times refers to occurrences; DIVIDE(COUNT(occurrences where w1st = w2nd), COUNT(occurrences)) as percentage;
SELECT CAST(COUNT(CASE WHEN w1st = w2nd THEN 1 ELSE NULL END) AS REAL) * 100 / COUNT(w1st) FROM biwords