在同一列上完全联接多个表 [英] Full join multiple tables on same columns

问题描述

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 ║
╚═══╩════╝ ╚═══╩════╝ ╚═══╩════╝ ╚═══╩════╝

我需要在K列中加入它们，然后选择V1，V2，V3和V4:

I need to join them by the column K and select V1, V2, V3 and 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    ║
╚═══╩══════╩══════╩══════╩══════╝

一种解决方法是

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;

但这仅适用于此简单示例，如果我有更多的Ks(连接条件)和Vs，则连接条件最终会变得更加复杂.

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.

如何改善此查询?

推荐答案

有几种方法，但是我认为以下是最受欢迎的方法:

There are several ways, but I think the following is the most popular:

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;

如果您有一个包含所有k值的单独表，则可以使用该表代替allk子查询.

If you have a separate table of all the k values, you can use that instead of the allk subquery.

编写full outer join查询的一种更简单的方法是使用coalesce():

An easier way to write your full outer join query is to use 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);

第三个方法是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;

这篇关于在同一列上完全联接多个表的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！