在LINQ中至少有两种方法可以进行LEFT OUTER JOIN
class Customer
{
public int ID { get; set; }
public string Name { get; set; }
}
class Order
{
public int ID { get; set; }
public string Product { get; set; }
}
static void Main()
{
// Example customers.
var customers = new Customer[]
{
new Customer{ID = 5, Name = "Sam"},
new Customer{ID = 6, Name = "Dave"},
new Customer{ID = 7, Name = "Julia"},
new Customer{ID = 8, Name = "Sue"},
new Customer{ID = 9, Name = "Joe"}
};
// Example orders.
var orders = new Order[]
{
new Order{ID = 5, Product = "Book"},
new Order{ID = 6, Product = "Game"},
new Order{ID = 7, Product = "Computer"},
new Order{ID = 8, Product = "Shirt"},
new Order{ID = 8, Product = "TShirt"}
};
// First Way
var query = from c in customers
join o in orders on c.ID equals o.ID into co
from x in co.DefaultIfEmpty()
where x != null && x.Product == "Shirt"
select new {c, x};
// Second Way
var query2 = from c in customers
from o in orders
where c.ID == o.ID && o.Product == "Shirt"
select new {c, o};
}
我找到了很多First Way的例子(使用'into'),这就是我以前做LEFT OUTER JOINS的方式。最近我找到了第二种方式,看起来更简单。我测试了,它们都产生了相同的结果。现在我的问题是存在任何隐藏的差异,表现,还是仅仅是语法糖?
非常感谢。
答案 0 :(得分:6)
如果您不使用!= null检查
,则应使用第一种方法左外连接选择左表中的所有内容以及右表中匹配的所有项,如果右表中不存在匹配,则仍返回结果,但右表中的值为null(linq转换为此作为集合项类型的默认值)
您的查询都有效地执行内部联接,因此它们将是相同的。
获得不同的结果
// First Way
var query = from c in customers
join o in orders on c.ID equals o.ID into co
from x in co.DefaultIfEmpty()
where c.Id>5
select new {c, x};
// Second Way
var query2 = from c in customers
from o in orders
where c.ID == o.ID && c.ID > 5
select new {c, o};