Sql 将具有共享键字段的多行查询为每个键的单个结果行

我有一个名为Form的表，其中包含以下内容：

    FormType KeyField  Field   AlphaValue            NumericValue
    -------- --------- ------- --------------------  -------------
    A        AAA001    A001    EXT                   0.000000
    A        AAA001    A002    mail                  0.000000
    A        AAA001    A003                          190.000000
    A        AAA001    A004    example@example.com   0.000000
    A        ABC123    A001    DIST                  0.000000
    A        ABC123    A002    something             0.000000
    A        ABC123    A003                          215.000000
    A        BBB255    A002    delivery              0.000000
    A        BBB255    A003                          94.000000
    A        CCC923    A002    TECH                  0.000000
    A        DDD123    A004    mail@example.com      0.000000

所需的结果需要是键字段，后跟每个可能的字段（在本例中为A001、A002、A003、A004），类似于：

    TypeA   A001         A002         A003        A004
    ------- ------------ ------------ ----------- --------------------
    AAA001  EXT          mail         190.000000  example@example.com
    ABC123  DIST         something    215.000000
    BBB255               delivery     94.000000
    CCC923  TECH                      0.000000
    DDD123                            0.000000    mail@example.com

我一直在努力，但没有真正反映出预期的结果：

    select
    a.KeyField as 'TypeA',
    b.AlphaValue as 'A001',
    c.AlphaValue as 'A002', 
    d.NumericValue as 'A003', 
    e.AlphaValue as 'A004'
    from (select distinct KeyField from Form where FormType = 'A') as a,
    (select AlphaValue, KeyField from Form where FormType = 'A' and Field = 'A001') as b,
    (select AlphaValue, KeyField from Form where FormType = 'A' and Field = 'A002') as c,
    (select NumericValue, KeyField from Form where FormType = 'A' and Field = 'A003') as d,
    (select AlphaValue, KeyField from Form where FormType = 'A' and Field = 'A004') as e
    where 
    b.KeyField = a.KeyField 
    and c.KeyField = a.KeyField 
    and d.KeyField = a.KeyField 
    and e.KeyField = a.KeyField

上一个查询结果为：

    TypeA   A001         A002         A003        A004
    ------- ------------ ------------ ----------- --------------------
    AAA001  EXT          mail         190.000000  example@example.com

当我需要将没有匹配项的必填字段替换为空字符串或0.000000时，它只跳过所有没有结果的键

---

关于如何达到预期的效果，你有什么想法吗？

试试下面的方法：

SELECT 
    master.KeyField AS 'TypeA',
    ISNULL(A001.AlphaValue, '') AS 'A001',
    ISNULL(A002.AlphaValue, '') AS 'A002', 
    ISNULL(A003.NumericValue, 0) AS 'A003', 
    ISNULL(A004.AlphaValue, '') AS 'A004'
FROM (
    SELECT FormType, KeyField
    FROM Form
    WHERE FormType = 'A'
    GROUP BY FormType, KeyField
) AS master
LEFT JOIN Form AS A001 
    ON master.FormType = A001.FormType AND master.KeyField = A001.KeyField AND A001.Field = 'A001'
LEFT JOIN Form AS A002 
    ON master.FormType = A002.FormType AND master.KeyField = A002.KeyField AND A002.Field = 'A002'
LEFT JOIN Form AS A003 
    ON master.FormType = A003.FormType AND master.KeyField = A003.KeyField AND A003.Field = 'A003'
LEFT JOIN Form AS A004 
    ON master.FormType = A004.FormType AND master.KeyField = A004.KeyField AND A004.Field = 'A004'

---

基本上，我们是在查询唯一键，然后左键连接所有其他字段。ISNULL函数提供默认值

如果您正在进行聚合（这是

distinct

的基本功能），那么您最好使用条件聚合：

select KeyField,
       max(case when keyfield = 'A001' then alphafield else '' end) as A001,
       max(case when keyfield = 'A002' then alphafield else '' end) as A002,
       max(case when keyfield = 'A003' then NumericValue else 0 end) as A003,
       max(case when keyfield = 'A004' then alphafield else ''end) as A004
from Form
where FormType = 'A'
group by KeyField;

---

这假设每个键只有一个值（如问题所暗示的）。

您需要学习如何使用连接而不是逗号分隔的查询。这种编码方式是对每个查询进行内部连接。当您使用EAV模式创建数据结构时，这是一个真正的问题。如果您可以将您的示例发布为易于使用的内容，我可以帮助您构建一个查询，从中获取数据。这篇文章是一个很好的开始。很好的黑客与最大：）但你需要添加组的人clause@PavelMorshenyuk . . . 非常感谢。