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
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)：
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
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
}