Something

SELECT * 
FROM (
	SELECT qgram.qg AS qg1, qgram.sid AS sid1, qgram.i AS i1, query.qid AS qid1, (qgram.i-query.qid) AS diag1 
	FROM qgram 
	INNER JOIN query 
	ON qgram.qg = query.qg) 
AS qgram1 
INNER JOIN (
	SELECT qgram.qg AS qg2, qgram.sid AS sid2, qgram.i AS i2, query.qid AS qid2, (qgram.i-query.qid) AS diag2 
	FROM qgram 
	INNER JOIN query 
	ON qgram.qg = query.qg) 
AS qgram2 
ON sid1 = sid2 
AND qg1 <> qg2 
AND i1 < i2 
AND diag1 = diag2;