DefaultIfEmpty and Left Outer Join

本文主要是介绍DefaultIfEmpty and Left Outer Join，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

http://www.xmaspx.com/u/guitarpark/Blog.aspx/t-19

  DefaultIfEmpty函数：若指定数据源为空的时候，返回一个默认值，此函数延时加载。
例：
int[] ints = { 1, 2, 3, 4, 5, 6 };var result = ints.DefaultIfEmpty();foreach (int x in result)Response.Write(x);
由于ints数组不为空，所以最后输出结果为123456。int[] ints = { 1, 2, 3, 4, 5, 6 };var result = ints.DefaultIfEmpty(-1);foreach (int x in result)Response.Write(x);
同上，由于ints数组不为空，所以最后输出结果为123456。int[] ints = { };var result = ints.DefaultIfEmpty(-1);foreach (int x in result)Response.Write(x);
由于ints数组为空，所以最后输出结果为ints1.DefaultIfEmpty(-1)中指定的-1。

http://www.cnblogs.com/cs_net/articles/2059846.html

左外连接与SqL中left join一样。如下例找出根据publisher中找出SampleData.Publishers中所有资料和book中存在于publisher的资料。

左外连接查询语句如下：

  
              // left join, 为空时用default 
              var leftJoinQuerybyDefault  =  from publisher  in  SampleData.Publishers
                                          join book  in  SampleData.Books
                                            on publisher equals book.Publisher into publisherBooks
                                          from book  in  publisherBooks.DefaultIfEmpty()
                                          select  new 
                                          {
                                              PublisherName  =  publisher.Name,
                                              BookName  =  (book  ==   default (Book))  ?   " no book "  : book.Title
                                          }; 
  

注：上例中使用了DefaultIfEmpty操作符，它能够为实序列提供一个默认的元素。DefaultIfEmpty使用了泛型中的default关键字。default关键字对于引用类型将返回null,而对于值类型则返回0。对于结构体类型，则会根据其成员类型将它们相应地初始化为null(引用类型)或0(值类型)。

我们可以不使用default关键字，但在要DefaultIfEmpty中给定当空时的默认对象值。语句如下：

  
              // left join, 为空时使用默认对象 
              var leftJoinQuery  =  from publisher  in  SampleData.Publishers
                                         join book  in  SampleData.Books
                                           on publisher equals book.Publisher into publisherBooks
                                         from book  in  publisherBooks.DefaultIfEmpty(
                                          new  Book { Title  =   ""  }                          // 设置为空时的默认值 
                                          )
                                         select  new 
                                         {
                                             PublisherName  =  publisher.Name,
                                             BookName  =  book.Title
                                         }; 