The Sonata for violin and keyboard in G minor, BWV 1020 is almost certainly not a work by Bach; or, rather, it is almost certainly not a work by J.S. Bach (it may in fact have been composed by Johann Sebastian's son C.P.E. Bach). Furthermore, it is not really even a violin sonata -- whoever the work's author might be, the intended ensemble seems actually to be flute and harpsichord (or perhaps its smaller-toned cousin the clavichord). But it is an elegant piece of late-Baroque chamber music, and is not put to any shame by its six worthy and unquestionably authentic brethren (BWV 1014 - 1019).
If the Sonata in G minor is the only one of the Bach-attributed violin/harpsichord sonatas to have three rather than four (or, in one case, five) movements. The opening movement has no tempo indication but is built of vintage allegro stock. The entirety of the opening ritornello, with its active figuration and arpeggiated subject, is given to the harpsichord as a solo; when the violin enters some bars later the music briefly takes on a more spacious form -- but soon the energetic ritornello creeps back in. The violin sings a melody that grows from many long-held tones in the Adagio second movement. The third movement is a strong-boned Allegro into which from time to time breaks a wonderfully peculiar repeated-note motif.