mysql join tables query
Here's one approach:

SELECT  a.part,
        a.description AS "desc",
        a.job_numbers
        dn.delnote_number,
        a.qty_ordered,
        dn.qty_del
    FROM ( SELECT  part, description, jobitemid
                GROUP_CONCAT(job_number ORDER BY job_number desc) AS job_numbers,
                SUM(qty_ordered) as qty_ordered FROM  job
            where  type = 'part'
            GROUP BY  part
         ) AS a
    LEFT JOIN dn  USING(jobitemid)


OP says that this is the result:

part        desc     job_numbers    delnote_num     qty_ordered      qty_del

"part1"     "desc1"    "2,1"       "1"              "456"            "30"
"part3"     "desc3"    "2"         "4"              "6666"           "6000"
"part1"     "desc1"    "2,1"       "5"              "456"            "500"
"part2"     "desc2"    "2,1"       NULL             "1343"           NULL
"part4"     "desc4"    "3"         NULL             "1000"           NULL