如何在LINQ中执行多个左连接、分组依据和连接

// 假设有两个表：Customers 和 Orders
// Customers 表包含以下列：CustomerId, CustomerName
// Orders 表包含以下列：OrderId, CustomerId, OrderDate

// 左连接示例
var query = from customer in Customers
            join order in Orders on customer.CustomerId equals order.CustomerId into customerOrders
            from co in customerOrders.DefaultIfEmpty()
            select new { customer.CustomerName, co?.OrderDate };

// 分组依据示例
var query = from order in Orders
            group order by order.CustomerId into g
            select new { CustomerId = g.Key, OrderCount = g.Count() };

// 连接示例
var query = from customer in Customers
            join order in Orders on customer.CustomerId equals order.CustomerId
            select new { customer.CustomerName, order.OrderDate };