SQL 完全连接同一列上的多个表
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Full join multiple tables on same columns
提问by pomber
I have four tables:
我有四个表：
╔═══╦════╗ ╔═══╦════╗ ╔═══╦════╗ ╔═══╦════╗
║ K ║ V1 ║ ║ K ║ V2 ║ ║ K ║ V3 ║ ║ K ║ V4 ║
╠═══╬════╣ ╠═══╬════╣ ╠═══╬════╣ ╠═══╬════╣
║ A ║ 1 ║ ║ B ║ 2 ║ ║ B ║ 3 ║ ║ C ║ 6 ║
║ C ║ 4 ║ ║ D ║ 7 ║ ║ C ║ 5 ║ ║ D ║ 8 ║
╚═══╩════╝ ╚═══╩════╝ ╚═══╩════╝ ╚═══╩════╝
I need to join them by the column K and select V1, V2, V3 and V4:
我需要通过 K 列加入它们并选择 V1、V2、V3 和 V4：
╔═══╦══════╦══════╦══════╦══════╗
║ K ║ V1 ║ V2 ║ V3 ║ V4 ║
╠═══╬══════╬══════╬══════╬══════╣
║ A ║ 1 ║ NULL ║ NULL ║ NULL ║
║ B ║ NULL ║ 2 ║ 3 ║ NULL ║
║ C ║ 4 ║ NULL ║ 5 ║ 6 ║
║ D ║ NULL ║ 7 ║ NULL ║ 8 ║
╚═══╩══════╩══════╩══════╩══════╝
One way to solve it is this:
解决它的一种方法是：
DECLARE @T1 TABLE (K VARCHAR(1), V1 INT);
DECLARE @T2 TABLE (K VARCHAR(1), V2 INT);
DECLARE @T3 TABLE (K VARCHAR(1), V3 INT);
DECLARE @T4 TABLE (K VARCHAR(1), V4 INT);
INSERT INTO @T1 VALUES ('A', 1), ('C', 4);
INSERT INTO @T2 VALUES ('B', 2), ('D', 7);
INSERT INTO @T3 VALUES ('B', 3), ('C', 5);
INSERT INTO @T4 VALUES ('C', 6), ('D', 8);
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 = t1.K OR t3.K = t2.K
FULL JOIN @T4 t4 ON t4.K = t1.K OR t4.K = t2.K OR t4.K = t3.K
ORDER BY 1;
But this works well only for this simple example, if I have more Ks (join conditions) and Vs, the join conditions end up being a lot more complex.
但这仅适用于这个简单的例子，如果我有更多的 Ks（连接条件）和 Vs，连接条件最终会变得更加复杂。
How can this query be improved?
如何改进此查询？
回答by Gordon Linoff
There are several ways, but I think the following is the most popular:
有几种方法，但我认为以下是最受欢迎的：
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;
If you have a separate table of all the kvalues, you can use that instead of the allksubquery.
如果您有一个包含所有k值的单独表，则可以使用它来代替allk子查询。
An easier way to write your full outer joinquery is to use coalesce():
编写full outer join查询的更简单方法是使用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);
And the third method is the union all/group bymethod:
第三种方法是union all/group by方法：
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;
