为什么这些连接查询返回重复记录？

Question

表结构：

    Field                Type           Null        Default
    --------------------------------------------------------------------
    OrderId               bigint(20)     Yes        II is a Primary key 
    CustomerID            bigint(20)     Yes        
    OrderDate             date           Yes        
    ShippedDate           date           Yes        NULL
    Freight               int(20)        Yes        NULL
    ShipName              varchar(50)    Yes        NULL
    ShipAddress           varchar(100)   Yes        NULL
    ShipCity              varchar(50)    Yes        NULL
    ShipPostalCode        int(20)        Yes        NULL
    ShipCountry           varchar(30)    Yes        NULL
    ShipVia               int(50)        Yes        1
    customerPayA          int(11)        Yes         
    discount              int(11)        Yes        
    shippingPackagingCost int(11)        Yes     
    grandTotal            int(11)        Yes        NULL
    remainPayment         int(20)        Yes        NULL
    Table structure for table orderdetails
    Field            Type            Null        Default
    ProductId        int(20)        Yes        
    OrderId          int(20)        Yes        
    UnitPrice        int(200)       Yes        
    Qty              int(20)        Yes        
    vat              int(11)        Yes        
    Amount           int(20)        Yes        
    Table structure for table products
    Field        Type        Null        Default
    ProductId     int(20)        Yes        
    ProductName   varchar(50)    Yes        
    Table structure for table shippers
    Field        Type        Null        Default
    ShipperID    int(11)       Yes        
    companyName  varchar(30)   Yes            
    Table structure for table customers
    Field               Type        Null        Default
    CustomerID        int(20)            Yes        
    firstName         varchar(20)        Yes        
    middleName        varchar(20)        Yes        
    LastName          varchar(20)        Yes        
    address           varchar(200)       Yes        
    city              varchar(20)        Yes        
    postalcode        int(10)            Yes        
    country           varchar(20)        Yes  

选择查询：

SELECT DISTINCT
    o.OrderID, o.CustomerID, 
    c.firstName as BillName , 
    s.CompanyName as ShipperName,  
    p.ProductName, od.UnitPrice, od.Qty, od.Amount,  
    o.grandTotal 
FROM 
    Orders o  
JOIN 
    Customers c ON o.CustomerID = c.CustomerID  
JOIN  
    Shippers s ON o.ShipVia = s.ShipperID  
JOIN 
    OrderDetails od ON o.OrderID = od.OrderID  
JOIN 
    Products p ON od.ProductID = p.ProductID  
WHERE 
    o.OrderID =46 

1. orderid下的订单表是主键
2. customerid 下的客户表是主键
3. shipperid 下的shippers 表是主键

[![sql select查询下的图片，输出方便理解][1]][1]

输出如下所示：

OrderID CustomerID         BillName Shipper Product UnitPrice         Qty Amount grandTotal

    46                  1              bharat         balaji         evergreent         400         4         1600         3885
    46                  1              bharat         balaji         evergreent         400         4         1600         3885
    46                  1              bharat         balaji         corogen         700         3         2100         3885
    46                  1              bharat         balaji         corogen         700         3         2100         3885

Answer 1

你写的查询很好，你使用了 Distinct 所以它不应该返回重复记录。

请检查您表格的原始数据。

应该有重复的数据，如空格或不可见字符的微小变化......

Answer 2

您正在寻找一个群组：

您的查询应如下所示：-

 "SELECT o.OrderID, o.CustomerID, o.OrderDate, o.ShippedDate, o.Freight, o.customerPayAmount, " +
                "o.ShipName, o.ShipAddress, o.ShipCity, o.ShipPostalCode, o.ShipCountry, " +
                "c.firstName as BillName, c.Address as BillAddress, " +
                "c.City as BillCity, c.PostalCode as BillPostalCode, " +
                "c.Country as BillCountry, s.CompanyName as ShipperName, " +
                "p.ProductName, od.UnitPrice, od.Qty,od.Amount, " +
                "o.discount, o.shippingPackagingCost, od.vat, o.grandTotal, o.remainPayment "+
                "FROM Orders o " +
                "JOIN Customers c ON " +
                "o.CustomerID = c.CustomerID " +
                "JOIN  Shippers s ON " +
                "o.ShipVia = s.ShipperID " +
                "JOIN OrderDetails od ON " +
                " o.OrderID = od.OrderID " +
                "JOIN Products p ON " +
                "od.ProductID = p.ProductID " +
                "WHERE o.OrderID = ? "+
                "group by o.OrderID,c.firstName,s.CompanyName,od.UnitPrice,p.ProductName");

Answer 3

您需要的是分组依据。如果您需要返回多条记录，您可能需要使用 GROUP_CONCAT 函数。

GROUP_CONCAT( p.ProductName )

查看开发者网站了解有关 GROUP_CONCAT 函数的更多信息：

GROUP_CONCAT - MYSQL