Der er flere måder, men jeg tror, at følgende er den mest populære:
select allk.k, t1.c1, t2.v2, t3.v3, t4.v4
from (select k from t1 union
select k from t2 union
select k from t3 union
select k from t4
) allk left outer join
t1
on allk.k = t1.k left outer join
t2
on allk.k = t2.k left outer join
t3
on allk.k = t3.k left outer join
t4
on allk.k = t4.k;
Hvis du har en separat tabel over alle k værdier, kan du bruge det i stedet for allk underforespørgsel.
En nemmere måde at skrive din full outer join forespørgslen skal bruge coalesce() :
SELECT COALESCE(t1.K, t2.K, t3.K, t4.K) [K], V1, V2, V3, V4
FROM @T1 t1 FULL JOIN
@T2 t2
ON t2.K = t1.K FULL JOIN
@T3 t3
ON t3.K = coalesce(t1.K, t2.K) FULL JOIN
@T4 t4
ON t4.K = coalesce(t1.K, t2.K, t3.k);
Og den tredje metode er union all /group by metode:
select k, max(v1) as v1, max(v2) as v2, max(v3) as v3, max(v4) as v4
from (select k, v1, NULL as v2, NULL as v3, NULL as v4
from t1 union all
select k, NULL as v1, v2, NULL as v3, NULL as v4
from t2 union all
select k, NULL as v1, NULL as v2, v3, NULL as v4
from t3 union all
select k, NULL as v1, NULL as v2, NULL as v3, v4
from t4
) t
group by k;