データの結合方法

SELECT 長さ,SUM(販売数)AS'販売数',SUM(在庫数)AS'在庫数' FROM (
(select 長さ,sum(販売数)as'販売数',0 AS '在庫数' from 販売 group by 長さ)
UNION
(select 長さ,0 AS '販売数',sum(在庫数)as'在庫数' from 在庫 group by 長さ)
)AS T GROUP BY 長さ

select z.長さ,sum(販売数)as'販売数',sum(在庫数)as'在庫数'
 from 在庫 z full outer join 販売 h on z.長さ=h.長さ
  group by z.長さ

SELECT 長さ, 在庫数, 販売数
FROM (
  SELECT 長さ,SUM(在庫数) FROM 在庫テーブル GROUP BY 長さ
) A
JOIN (
  SELECT 長さ,SUM(販売数) FROM 販売テーブル GROUP BY 長さ 
) B
ON A.長さ = B.長さ
ORDER BY
  長さ

select
  coalesce(Z.長さ,H.長さ) as 長さ,
  coalesce(sum(在庫数),0) as 在庫数,
  coalesce(sum(販売数),0) as 販売数
 from 在庫t as Z
  full join 販売t as H
   on Z.長さ=H.長さ
 group by Z.長さ,H.長さ
 order by 1