mysql - SQL查询按时间顺序获取不同表的两列的组合

Question

---------------------------------
| table_one.col_a| table_one.fk |
---------------------------------
| A              | 4            |
---------------------------------
| B              | 4            |
---------------------------------
| C              | 4            |
---------------------------------

---------------------------------
| table_two.col_b|table_two.fk  |
---------------------------------
| E              | 4            |
---------------------------------
| F              | 4            |
---------------------------------
| G              | 4            |
---------------------------------
| H              | 4            |
---------------------------------

我正在寻找一个查询，当 fk 是通用外键时，它会输出类似这样的内容

 A_E
 B_F
 C_G
  _H

Answer 1

您需要使用变量来创建计数器字段:

例如

SELECT  col_a,
        fk,
        @r:= CASE WHEN @FK = fk THEN @r + 1 ELSE 1 END AS RowNumber,
        @fk:= fk
FROM    table_one, 
        (SELECT @fk:= 0) fk,
        (SELECT @r:= 0) r
ORDER BY fk, col_a;

会回来

+-------+----+-----------+
| col_a | fk | RowNumber |
+-------+----+-----------+
|  A    | 4  |    1      |
|  B    | 4  |    2      |
|  C    | 4  |    3      |
+-------+----+-----------+

在这里，您必须首先按分区字段排序，例如如果您希望每个 fk 的行号重置为 1，您必须首先按此顺序排序。然后您的下一个顺序决定了 RowNumber 的应用顺序。因此，如果您更改为:

ORDER BY fk, col_a DESC;

你会得到:

+-------+----+-----------+
| col_a | fk | RowNumber |
+-------+----+-----------+
|  C    | 4  |    1      |
|  B    | 4  |    2      |
|  A    | 4  |    3      |
+-------+----+-----------+

请注意，A 和 C 的行号现在不同

因此，如果您对两个表执行相同的操作，则可以加入 fk 和 RowNumber:

SELECT  b.fk, CONCAT(COALESCE(a.col_a, ''), '_', COALESCE(b.Col_b, '')) AS ColA_ColB
FROM    (   SELECT  col_b,
                    fk,
                    @r:= CASE WHEN @FK = fk THEN @r + 1 ELSE 1 END AS RowNumber,
                    @fk:= fk
            FROM    table_two, 
                    (SELECT @fk:= 0) fk,
                    (SELECT @r:= 0) r
            ORDER BY fk, col_b
        ) b
        LEFT JOIN
        (   SELECT  col_a,
                    fk,
                    @r:= CASE WHEN @FK = fk THEN @r + 1 ELSE 1 END AS RowNumber,
                    @fk:= fk
            FROM    table_one, 
                    (SELECT @fk:= 0) fk,
                    (SELECT @r:= 0) r
            ORDER BY fk, col_a
        ) a
            ON b.fk = a.fk
            AND b.RowNumber = a.RowNumber;

<强> Example on SQL Fiddle

如果您不知道哪个表将包含更多记录，由于 MySQL 不支持完全连接，查询会变得更加复杂，您需要执行 UNION ALL 来合并数据集:

SELECT  a.fk,
        a.RowNumber,
        CONCAT(MAX(CASE WHEN TableSource = 1 THEN Col ELSE '' END), 
                '_',
                MAX(CASE WHEN TableSource = 2 THEN Col ELSE '' END)) AS Col_AB
FROM    ((  SELECT  col_b AS Col,
                    fk,
                    @r:= CASE WHEN @FK = fk THEN @r + 1 ELSE 1 END AS RowNumber,
                    @fk:= fk,
                    2 AS TableSource
            FROM    table_two, 
                    (SELECT @fk:= 0) fk,
                    (SELECT @r:= 0) r
            ORDER BY fk, col_b
        )
        UNION ALL
        (   SELECT  col_a,
                    fk,
                    @r:= CASE WHEN @FK = fk THEN @r + 1 ELSE 1 END AS RowNumber,
                    @fk:= fk,
                    1 AS TableSource
            FROM    table_one, 
                    (SELECT @fk:= 0) fk,
                    (SELECT @r:= 0) r
            ORDER BY fk, col_a
        )) a
GROUP BY a.fk, a.RowNumber;

<强> Example on SQL Fiddle

<小时/>

编辑

感谢Andriy M ，这两个查询可以整理如下:

• 加入方法:

  SELECT  b.fk, CONCAT(COALESCE(a.col_a, ''), '_', COALESCE(b.Col_b, '')) AS ColA_ColB
  FROM    (   SELECT  col_b,
                      @r:= (@fk = fk) * @r + 1 AS RowNumber,
                      @fk:= fk AS fk
              FROM    table_two, 
                      (SELECT @fk:= 0, @r:= 0) r
              ORDER BY fk, col_b
          ) b
          LEFT JOIN
          (   SELECT  col_a,
                      @r:= (@fk = fk) * @r + 1 AS RowNumber,
                      @fk:= fk AS fk
              FROM    table_one, 
                      (SELECT @fk:= 0, @r:= 0) r
              ORDER BY fk, col_a
          ) a
              ON b.fk = a.fk
              AND b.RowNumber = a.RowNumber;
  

• 联合方法:

  SELECT  a.fk,
          a.RowNumber,
          CONCAT(MAX(CASE WHEN TableSource = 1 THEN Col ELSE '' END), 
                  '_',
                  MAX(CASE WHEN TableSource = 2 THEN Col ELSE '' END)) AS Col_AB
  FROM    ((  SELECT  col_b AS Col,
                      @r:= (@fk = fk) * @r + 1 AS RowNumber,
                      @fk:= fk AS fk,
                      2 AS TableSource
              FROM    table_two, 
                      (SELECT @fk:= 0, @r:= 0) s
              ORDER BY fk, col_b
          )
          UNION ALL
          (   SELECT  col_a,
                      @r:= (@fk = fk) * @r + 1 AS RowNumber,
                      @fk:= fk AS fk,
                      1 AS TableSource
              FROM    table_one, 
                      (SELECT @fk:= 0, @r:= 0) s
              ORDER BY fk, col_a
          )) a
  GROUP BY a.fk, a.RowNumber;
  

<强> Examples on SQL Fiddle