单个字段的子查询创建多个记录

Question

我的场景：一个请求可以有多个（最多 3 个）拒绝代码。第一个是主要拒绝代码，次要（第二和第三）拒绝代码是可选的。要确定 rejection_code 排名，有一个名为 rejection_rowId 的字段。如果 rejection_rowId = '1'，则为初级拒绝。如果rejection_rowId = '2'，则为第二次拒绝。如果是'3'，就是第三个。现在，我试图在一条记录中显示属于一个请求的所有 rejection_code 值，而不是为每个 rejection_code 存储 1 条记录。我运行遇到了麻烦，想知道是否有人可以解释为什么我的 SQL 无法正常工作。

我的SQL基本上是这样的；

SELECT DISTINCT rea.id
    ,(SELECT rejection_code 
      FROM rejection t1
      INNER JOIN reason t2 ON t1.rowId = t2.rowId
      WHERE t1.rejection_rowId = '1'
        AND t1.rowId = REA.rowId) AS rejection1

    ,(SELECT rejection_code 
      FROM rejection t1
      INNER JOIN reason t2 ON t1.rowId = t2.rowId
      WHERE t1.rejection_rowId = '2'
        AND t1.rowId = REA.rowId) AS rejection2

    ,(SELECT rejection_code 
      FROM rejection t1
      INNER JOIN reason t2 ON t1.rowId = t2.rowId
      WHERE t1.rejection_rowId = '3'
        AND t1.rowId = REA.rowId) AS rejection3

FROM reason rea
INNER JOIN rejection rej ON rea.rowId = rej.rowId
WHERE rea.id = '12345';

上面SQL生成的输出是...

id    | rejection1 | rejection2 | rejection3
12345 | 26         | NULL       | NULL
12345 | NULL       | 51         | NULL
12345 | NULL       | NULL       | 3

我尝试（但失败）生成的输出是...

id    | rejection1 | rejection2 | rejection3
12345 | 26         | 51         | 3

如果有人能帮我找出并解决问题，我将不胜感激。谢谢！

Answer 1

我认为你正在尝试做这样的事情：

SELECT
  rea.id,
  MAX(CASE WHEN rej.rejection_rowId = '1' THEN rej.rejection_code END) AS rejection1,
  MAX(CASE WHEN rej.rejection_rowId = '2' THEN rej.rejection_code END) AS rejection2,
  MAX(CASE WHEN rej.rejection_rowId = '3' THEN rej.rejection_code END) AS rejection3,
FROM
  reason rea
  INNER JOIN rejection rej ON rea.rowId = rej.rowId
WHERE
  rea.id = '12345'
GROUP BY rea.id

不需要一次又一次地连接相同的表，使用子查询来实现你所需要的根本没有必要，而且成本很高。

使用 GROUP BY 子句可以将结果限制为一行。使用聚合函数内的条件，您仅在实际存在拒绝时才在列中显示拒绝，并且您可以将它们拆分为单独的列。

Answer 2

试试这个：

求和：

SELECT DISTINCT rea.id
    ,SUM(SELECT rejection_code 
      FROM rejection t1
      INNER JOIN reason t2 ON t1.rowId = t2.rowId
      WHERE t1.rejection_rowId = '1'
        AND t1.rowId = REA.rowId) AS rejection1

    ,SUM(SELECT rejection_code 
      FROM rejection t1
      INNER JOIN reason t2 ON t1.rowId = t2.rowId
      WHERE t1.rejection_rowId = '2'
        AND t1.rowId = REA.rowId) AS rejection2

    ,SUM(SELECT rejection_code 
      FROM rejection t1
      INNER JOIN reason t2 ON t1.rowId = t2.rowId
      WHERE t1.rejection_rowId = '3'
        AND t1.rowId = REA.rowId) AS rejection3

FROM reason rea
INNER JOIN rejection rej ON rea.rowId = rej.rowId
WHERE rea.id = '12345'
GROUP BY rea.id ;

最大值：

SELECT DISTINCT rea.id
    ,MAX(SELECT rejection_code 
      FROM rejection t1
      INNER JOIN reason t2 ON t1.rowId = t2.rowId
      WHERE t1.rejection_rowId = '1'
        AND t1.rowId = REA.rowId) AS rejection1

    ,MAX(SELECT rejection_code 
      FROM rejection t1
      INNER JOIN reason t2 ON t1.rowId = t2.rowId
      WHERE t1.rejection_rowId = '2'
        AND t1.rowId = REA.rowId) AS rejection2

    ,MAX(SELECT rejection_code 
      FROM rejection t1
      INNER JOIN reason t2 ON t1.rowId = t2.rowId
      WHERE t1.rejection_rowId = '3'
        AND t1.rowId = REA.rowId) AS rejection3

FROM reason rea
INNER JOIN rejection rej ON rea.rowId = rej.rowId
WHERE rea.id = '12345'
GROUP BY rea.id ;

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