sql - 如果其他列值重复，如何在第一次出现后进行 1 列为空的 sql 查询？

Question

假设我们有来自表 invoice_details 的 4 列

Customer Order ||   invoice number.    ||   Work Order     ||    Invoice Amount
    O2911               9011                   20113                  5000
    O2911               9012                   20114                  6000
    O2912               9012                   20114                  6000
    O2913               9013                   20115                  7000
    O2914               9014                   20116                  6000

输出应该像

Customer Order ||   invoice number.    ||   Work Order     ||    Invoice Amount
    O2911         9011        20113        5000
    O2911         9012        20114        6000
    O2912         9012        20114          -
    O2913         9013        20115        7000
    O2914         9014        20116        6000

由于发票编号（9012）重复，其发票金额也是（即6000），所以我只想要发票编号的第一张发票金额，其余的都应该为空，如上所示。

Answer 1

如果您的 RDBMS 可以支持窗口函数，这可能很容易，

SELECT  "CustomerOrder", 
        "Invoicenumber", 
        "WorkOrder", 
        CASE WHEN rn = 1 
             THEN CAST("InvoiceAmount" AS VARCHAR(20))
             ELSE '-'
        END "InvoiceAmount"
FROM
        (
            SELECT  "CustomerOrder", "Invoicenumber", "WorkOrder", 
                    "InvoiceAmount",
                    ROW_NUMBER() OVER (PARTITION BY "Invoicenumber" 
                                        ORDER BY "CustomerOrder") rn
            FROM    TableName
        ) 

• SQLFiddle 演示

输出

╔═══════════════╦═══════════════╦═══════════╦═══════════════╗
║ CUSTOMERORDER ║ INVOICENUMBER ║ WORKORDER ║ INVOICEAMOUNT ║
╠═══════════════╬═══════════════╬═══════════╬═══════════════╣
║ O2911         ║          9011 ║     20113 ║ 5000          ║
║ O2911         ║          9012 ║     20114 ║ 6000          ║
║ O2912         ║          9012 ║     20114 ║ -             ║
║ O2913         ║          9013 ║     20115 ║ 7000          ║
║ O2914         ║          9014 ║     20116 ║ 6000          ║
╚═══════════════╩═══════════════╩═══════════╩═══════════════╝