来自多个表的SQL半连接

Question

我对以下查询下表有疑问

发票没有|协议号

  1     |     0       |
  2     |     0       |
  3     |     101     |
  4     |     102     |
  5     |     201     |
  1     |     201     |

每日协议表 -

协议号|协议状态

  101   |     closed      |
  102   |     open        |

月度协议表 -

协议号|协议状态

  201   |     closed      |
  202   |     open        |

我想创建一个查询来选择协议号和协议状态

的发票

请帮助

Answer 1

您可以使用左连接实现此目的。用你的替换这些表和列别名，这将是代码：

SELECT
      InvoiceNo
    , A.AgreementNo
    , [AgreementStatus] = IFNull(B.AgreementStatus, C.AgreementStatus)
FROM
    Agreements AS A
        LEFT JOIN
    DailyAgreements AS B
        ON A.AgreementNo = B.AgreementNo
        LEFT JOIN
    MonthlyAgreements AS C
        ON A.AgreementNo = C.AgreementNo

Answer 2

使用LEFT JOIN加入状态表，并使用CASE检查每月或每日状态表是否已加入：

SELECT 
    i.invoice_no
    , i.agreement_no
    , CASE WHEN da.agreement_status IS NULL THEN ma.agreement_status ELSE da.agreement_status END as agreement_status
FROM invoice as i
LEFT JOIN daily_agreement as da ON da.agreement_no = i.agreement_no
LEFT JOIN monthly_agreement as ma ON ma.agreement_no = i.agreement_no

修改

我的查询应该返回如下行：

invoice_no  | agreement_no| agreement_status
      1     |     0       | null
      2     |     0       | null
      3     |     101     | closed
      4     |     102     | open
      5     |     201     | closed
      1     |     201     | closed