The following query returns a totally bogus result – the rows returned can not be found in the original table.

CREATE TABLE t1 ( f1 int NOT NULL , PRIMARY KEY (f1)) ;
INSERT IGNORE INTO t1 VALUES (10),(11),(12),(13),(14),(15),(16),(17),(18),(19),(20);
SELECT SQL_BIG_RESULT f1 AS field1 , f1 AS field2
FROM t1
GROUP BY field1 , field2 ;

correct result:

SELECT f1 AS field1 , f1 AS field2
FROM t1
GROUP BY field1 , field2 ;