SELECT CONCAT_WS(',', NULLIF(`c1`,''),NULLIF(`c2`,''),NULLIF(`c3`,''),NULLIF(`c4`,''),NULLIF(`c5`,''),NULLIF(`c6`,''))
FROM `tb`
下記のような結果が出る
a,b,c
d,e
f,g,h,i,j
k
カラムにNULLを可にしていて、空欄とNULLを分けている場合はNULLIFを使わずにカラム名だけにする
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