Question

我有一个返回多条记录的查询，因为我使用的是

    SELECT m_o.ordernum, m_CD.fieldvalue
    FROM measurements.orders m_o
    INNER JOIN measurements.custom_data m_cd
    ON m_cd.ordernum = m_o.ordernum
    WHERE m_o.custnum = 'xxx'
    AND (m_cd.fieldname = 'primary_name' or m_cd.fieldname = 'secondary_name');

有一种方法可以让这一行返回，但我很难过。

Answer 1

您可以使用case然后在ordernum上进行汇总，因为您的字段名最多只有2个值。

SELECT m_o.ordernum, 
max(case when m_cd.fieldname = 'primary_name' then m_CD.fieldvalue end) as col1,
max(case when m_cd.fieldname = 'secondary_name' then m_CD.fieldvalue end) as col2
FROM measurements.orders m_o
INNER JOIN measurements.custom_data m_cd
ON m_cd.ordernum = m_o.ordernum
WHERE m_o.custnum = 'xxx'
group by m_o.ordernum

Answer 2

要获取与主要名称（如果有）相关联的值以及与同一结果行中的辅助名称（如果有）关联的值，您可以加入measurements.custom_data两次。如果您不能依赖这些名称中的哪一个具有与之关联的值，则必须使用外部联接。例如：

SELECT
  m_o.ordernum,
  m_cd1.fieldvalue AS fieldValue1
  m_cd2.fieldvalue AS fieldValue2
FROM
  measurements.orders m_o
  LEFT JOIN measurements.custom_data m_cd1
    ON m_cd1.ordernum = m_o.ordernum AND m_cd1.fieldname = 'primary_name'
  LEFT JOIN measurements.custom_data m_cd2
    ON m_cd2.ordernum = m_o.ordernum AND m_cd2.fieldname = 'secondary_name'
WHERE
  m_o.custnum = 'xxx'
  AND (m_cd1.ordernum IS NOT NULL OR m_cd2.ordernum IS NOT NULL)

请注意，fieldname谓词将移入（外部）连接条件。这指示哪个字段值进入哪个结果列，并有助于防止重复。

还请注意NOT NULL子句中的WHERE条件。这些会阻止返回的结果 名称无效。

SQL如何在一条记录中获取OR语句

2 个答案: