C# C中的SELECT语句#如果为Null则返回（Visual Studio）

Select语句仅返回不为空的技师名称。由于技术人员还没有分配到打开票据，我需要它返回所有产品代码，而不管技术名称是否已填写或为空

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";

更改此代码

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

对此

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

更改此代码

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

对此

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

---

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";

---

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";

---

如果您使用

JOIN

您实际上使用了

internaljoin

如果您使用

JOIN

您实际上使用了

internaljoin