SELECT  
fanny_itineraire.num_quai_depart
,fanny_itineraire.num_quai_arrive
,IF(section.id_section=tonnages_depart_troncons.id_section_1,tonnages_depart_troncons.troncon_1,tonnages_depart_troncons.troncon_2) as troncon
FROM fanny_itineraire
JOIN fanny_quai ON fanny_itineraire.num_quai_depart=fanny_quai.num_quai									
JOIN `section` on `section`.id_section=fanny_itineraire.num_section
JOIN tonnages_depart_troncons ON (fanny_itineraire.num_quai_arrive=tonnages_depart_troncons.num_quai_arrive AND fanny_itineraire.num_quai_depart=tonnages_depart_troncons.num_quai_depart)
WHERE fanny_itineraire.type_section='depart'