JOIN 每行仅一次——排列（分配）行 1 到 1

Question

我有两个 table 要加入，我希望每行只加入一次。这是示例数据：

CREATE TABLE A (id smallint, val varchar(1) );
CREATE TABLE B (id smallint, val varchar(1) );
INSERT INTO A VALUES (1, 'a'), (2, 'b'), (3, 'c'), (3, 'd');
INSERT INTO B VALUES (2, 'x'), (3, 'y'), (4, 'z'), (3, 'k');

当我们加入 id 时，我们获得：

mysql> SELECT * FROM A JOIN B ON A.id = B.id;
+------+------+------+------+
| id   | val  | id   | val  |
+------+------+------+------+
|    2 | b    |    2 | x    |
|    3 | c    |    3 | y    |
|    3 | d    |    3 | y    |
|    3 | c    |    3 | k    |
|    3 | d    |    3 | k    |
+------+------+------+------+

我想要的是：

+------+------+------+------+            +------+------+------+------+
| id   | val  | id   | val  |            | id   | val  | id   | val  |
+------+------+------+------+     or     +------+------+------+------+
|    2 | b    |    2 | x    |            |    2 | b    |    2 | x    |
|    3 | c    |    3 | y    |            |    3 | d    |    3 | y    |
|    3 | d    |    3 | k    |            |    3 | c    |    3 | k    |
+------+------+------+------+            +------+------+------+------+

顺序和排列无关紧要。

可能吗？怎么样？

根据 this answer 我需要指定如何 select 匹配行。在那种情况下，我想如果加入的 table 行已经被使用，则需要检查子查询；或一种与 id 相关的计数器......但我不知道如何写这个。

编辑：

澄清一下，我希望将 id 为 3 的每一行映射到连接 table 中的另一行，例如每一行只映射一次（我也很想知道当具有相同 id 的行在两个 tables 中不同):

(3, c) -> (3, y) [join only with the first row such as B.id = 3]
(3, d) -> (3, k) [the first row has been used, so map with (and only with) the second row such as B.id = 3]

但正如我所说，映射可以按任何其他顺序进行（例如，以相反的顺序映射行）。

Answer 1

我终于做到了！

SELECT T.ID_A,
       T.VAL_A,
       T.XXXX,
       T.ID_B,
       T.VAL_B,
       T.YYYY
FROM (

SELECT A.id AS ID_A,
       A.VAL AS VAL_A,
       ROW_NUMBER() OVER (PARTITION BY A.ID, A.VAL ORDER BY A.ID, A.VAL) AS XXXX,
       B.ID AS ID_B,
       B.VAL AS VAL_B,
       ROW_NUMBER() OVER (PARTITION BY B.ID, B.VAL ORDER BY B.ID DESC, B.VAL) AS YYYY
FROM A INNER JOIN B ON A.id = B.id) AS T
WHERE T.YYYY = 1

Answer 2

多亏了这个blog post:

SELECT A2.id, A2.val, B2.val FROM (
    SELECT l.id, l.val, COUNT(*) AS n1 FROM A AS l JOIN A AS r ON l.id = r.id AND l.val >= r.val GROUP BY l.id, l.val
) AS A2 JOIN (
    SELECT l.id, l.val, COUNT(*) AS n2 FROM B AS l JOIN B AS r ON l.id = r.id AND l.val >= r.val GROUP BY l.id, l.val
) AS B2 ON
A2.id = B2.id AND n1 = n2;

结果是：

+------+------+------+
| id   | val  | val  |
+------+------+------+
|    2 | b    | x    |
|    3 | c    | k    |
|    3 | d    | y    |
+------+------+------+

Answer 3

SQL Fiddle

MySQL 5.6 架构设置:

CREATE TABLE A (id smallint, val varchar(1) );
CREATE TABLE B (id smallint, val varchar(1) );
INSERT INTO A VALUES (1, 'a'), (2, 'b'), (3, 'c'), (3, 'd');
INSERT INTO B VALUES (2, 'x'), (3, 'y'), (4, 'z'), (3, 'k');

查询 1:

select
        aa.id  as aid
      , aa.val as aval
      , bb.id  as bid
      , bb.val as bval
from (
      select
            @row_num :=IF(@prev_value=a.id,@row_num+1,1)AS RowInGroup
          , a.id
          , a.val
          , @prev_value := a.id
      from (
            SELECT id, val 
            FROM A
            group by id, val
            /* order by ?? */
              ) a
            CROSS JOIN (
                        SELECT @row_num :=1,  @prev_value :=''
                       ) vars
      ) aa
INNER JOIN (
          select
                @row_num :=IF(@prev_value=b.id,@row_num+1,1)AS RowInGroup
              , b.id
              , b.val
              , @prev_value := b.id
          from (
                SELECT id, val 
                FROM B
                group by id, val
                /* order by ?? */
                  ) b
                CROSS JOIN (
                            SELECT @row_num :=1,  @prev_value :=''
                           ) vars
          ) bb on aa.id = bb.id and aa.RowInGroup = bb.RowInGroup
order by
        aa.id
      , aa.val

Results:

| id | val | id | val |
|----|-----|----|-----|
|  2 |   b |  2 |   x |
|  3 |   c |  3 |   k |
|  3 |   d |  3 |   y |

nb：您可以通过在计算序列 RowInGroup 的 group by id, val 的子查询中引入 order by 来影响最终结果。

JOIN 每行仅一次——排列（分配）行 1 到 1

JOIN each row only once — arrange (distribute) rows 1 to 1

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