Complex sentence: One of Benny Goodman's closest friends off and on, from the 1930s onward was celebrated Columbia records producer John H. Hammond, who influenced Goodman's move from RCA to the newly created Columbia records in 1939.
Split into simpler sentences: One of Benny Goodman's closest friends off and on, from the 1930s onward was celebrated Columbia records producer John H. Hammond. Hammond and Goodman were so close that Hammond influenced Goodman's move from RCA records to the newly created Columbia records in 1939.