我有一些麻烦，弄清楚如何使用LINQ to sql使用多个左外连接。我理解如何使用一个左外连接。我使用VB.NET。下面是我的sql语法。
T-sql
@H_502_3@SELECT o.OrderNumber,v.VendorName,s.StatusName FROM Orders o LEFT OUTER JOIN Vendors v ON v.Id = o.VendorId LEFT OUTER JOIN Status s ON s.Id = o.StatusId WHERE o.OrderNumber >= 100000 AND o.OrderNumber <= 200000
这可能更干净(你不需要所有的into语句)：
@H_502_3@var query =
from order in dc.Orders
from vendor
in dc.Vendors
.Where(v => v.Id == order.VendorId)
.DefaultIfEmpty()
from status
in dc.Status
.Where(s => s.Id == order.StatusId)
.DefaultIfEmpty()
select new { Order = order,Vendor = vendor,Status = status }
//Vendor and Status properties will be null if the left join is null
这里是另一个左连接示例
@H_502_3@var results = from expense in expenseDataContext.ExpenseDtos where expense.Id == expenseId //some expense id that was passed in from category // left join on categories table if exists in expenseDataContext.CategoryDtos .Where(c => c.Id == expense.CategoryId) .DefaultIfEmpty() // left join on expense type table if exists from expenseType in expenseDataContext.ExpenseTypeDtos .Where(e => e.Id == expense.ExpenseTypeId) .DefaultIfEmpty() // left join on currency table if exists from currency in expenseDataContext.CurrencyDtos .Where(c => c.CurrencyID == expense.FKCurrencyID) .DefaultIfEmpty() select new { Expense = expense,// category will be null if join doesn't exist Category = category,// expensetype will be null if join doesn't exist ExpenseType = expenseType,// currency will be null if join doesn't exist Currency = currency }