left使用linq连接2个表

在linq中使用左联接，如下所示。。。。

join t in Web
on websites.WebsiteID equals t.WebsiteID
into wt1
from wt in wt1.DefaultIfEmpty()

在此之后，wt将使用i其中的条件和select语句。。。。。。

通过使用这个概念，您可以在LINQ中进行左联接查询。。。。。

    ruleSets = (from rs in db.RuleSets
                join brs in db.BatchRuleSets on rs.ID equals brs.RuleSetID into j1
                from jbrs in j1.DefaultIfEmpty()
                join b in db.Batches on jbrs.BatchID equals b.id into j2
                from jb in j2.DefaultIfEmpty()
                where !clientId.HasValue || jb.ClientID == clientId.Value
                where !batchId.HasValue || jbrs.BatchID == batchId.Value
                select new
                {
                    rs.ID,
                    rs.Description,
                    rs.IsActive,
                    rs.CreatedDate,
                    rs.EffectiveDate,
                    rs.ExpirationDate,
                    BatchName = jb.FileName,
                    jb.ClientID
                }).ToList().Select(x => new { 
                    x.ID,
                    x.Description,
                    x.IsActive,
                    x.CreatedDate,
                    x.EffectiveDate,
                    x.ExpirationDate,
                    x.BatchName,
                    ClientName = GetClientName(x.ClientID)});

void Main()
{
    var customers = new List<Customer> {new Customer() { CustomerId = 1}, new Customer() { CustomerId = 2}};
    var orders = new List<Order> {new Order() { OrderId = 1, CustomerId = 1}, new Order() { OrderId = 2, CustomerId = 1}};
    var items = new List<Item> {new Item() { ItemId = 1, OrderId = 1}, new Item() { ItemId = 2, OrderId = 1}, new Item() { ItemId = 3, OrderId = 2}};
    var doubleJoin = from customer in customers
    join order in orders on customer.CustomerId equals order.CustomerId
    into customerOrders
    from co in customerOrders.DefaultIfEmpty()
    where (co != null)
    join item in items on co.OrderId equals item.OrderId
    select new {Customer = customer, Orders = co,  Items = item};
    doubleJoin.Dump();
}
public class Customer
{
    public int CustomerId { get; set; }
}
public class Order
{
    public int OrderId { get; set; }
    public int CustomerId { get; set;}
}
public class Item
{
    public int ItemId { get; set; }
    public int OrderId { get; set; }
}