C＃中的SELECT语句返回如果为空（Visual Studio）

Question

Select语句仅返回非空的技术人员姓名。由于技术人员尚未分配打开机票，我需要此功能返回所有产品代码，无论技术名称是否填充或为空。

string selectStatement =
            "SELECT ProductCode, DateOpened, Customers.Name AS CustName, " + 
            "Technicians.Name as TechName, Title, DateClosed " +
            "FROM Incidents " +
            "JOIN Customers ON Incidents.CustomerID = Customers.CustomerID " +
            "JOIN Technicians ON Incidents.TechID = Technicians.TechID " +
            "WHERE DateClosed IS NULL " +
            "ORDER BY DateOpened";

Answer 1

更改此代码

"JOIN Technicians ON Incidents.TechID = Technicians.TechID " +

到这个

"LEFT JOIN Technicians ON Incidents.TechID = Technicians.TechID " +

Answer 2

您可以使用如下所示的内部联接：

"inner join Technicians ON Incidents.TechID = Technicians.TechID " 

最终查询

 string selectStatement =
        "SELECT ProductCode, DateOpened, Customers.Name AS CustName, " + 
        "Technicians.Name as TechName, Title, DateClosed " +
        "FROM Incidents " +
        "inner JOIN Customers ON Incidents.CustomerID = Customers.CustomerID " +
        "inner JOIN Technicians ON Incidents.TechID = Technicians.TechID " +
        "WHERE DateClosed IS NULL " +
        "ORDER BY DateOpened";