SELECT polygon.id,
polygon.lib, -- Place here any field releveant for you (they must also be in grouping clauses, see below)
group_concat(line.id,',') as list_id_line -- this function concatenate the id of every line that touch you polygon
FROM polygon LEFT OUTER JOIN line
ON Intersects(polygon.geom,line.geom) -- Spatial Dabatabase Rule !
GROUP BY polygon.id, polygon.lib -- theses are the grouping clauses