What's the occurrence of the biwords pair whose first word is "àbac" and second word is "xinès"?
àbac refers to word = 'àbac'; xinès refers to word = 'xinès'; occurrence refers to occurrences
SELECT occurrences FROM biwords WHERE w1st = ( SELECT wid FROM words WHERE word = 'àbac' ) AND w2nd = ( SELECT wid FROM words WHERE word = 'xinès' )