LINQ中的连接(join)用法示例

Posted 2020-09-14 孔小爽

Linq中连接主要有组连接、内连接、左外连接、交叉连接四种。各个用法如下。

1、 组连接

    组连接是与分组查询是一样的。即根据分组得到结果。 如下例,根据publisther分组得到结果。

    使用组连接的查询语句如下：

 //使用组连接
            var GroupQuery = from publisher in SampleData.Publishers
                             join book in SampleData.Books
                                  on publisher equals book.Publisher into publisherBooks
                             select new
                             {
                                 PublisherName = publisher.Name,
                                 Books = publisherBooks
                             };

与上边等同的GroupBy语句如下：

//使用Group
            var QueryByGroup = from book in SampleData.Books
                        group book by book.Publisher into grouping
                        select new
                        {
                            PublisherName = grouping.Key.Name,
                            Books = grouping
                        };

2、内连接

    内连接与SqL中inner join一样，即找出两个序列的交集。如下例找出book中的Publisher存在于SampleData.Publishers的资料。

    内连接查询语句如下：

//join查询语句
            var joinQuery = from publisher in SampleData.Publishers
                            join book in SampleData.Books
                                on publisher equals book.Publisher
                            select new
                            {
                                PublisherName = publisher.Name,
                                BookName = book.Title
                            };

与上边等同的查询操作符语句如下：

//join操作符语句
            SampleData.Publishers.Join(
                SampleData.Books,               //join 对象
                publisher => publisher,         //外部的key
                book => book.Publisher,         //内部的key
                (publisher, book) => new        //结果
                {
                    PublisherName = publisher.Name,
                    BookName = book.Title
                });

3、左外连接

     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)) ? "" : book.Title //这里主要第二个集合有可能为空。需要判断
                                         };

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
                                        };

4、交叉连接

    交叉连接与SqL中Cross join一样。如下例中找出SampleData.Publishers与SampleData.Books的交叉连接。

    交叉连接查询语句：

var crossJoinQuery = from publisher in SampleData.Publishers
                                 from book in SampleData.Books
                                 select new
                                 {
                                     PublisherName = publisher.Name,
                                     BookName = book.Title
                                 };

    查询操作符语句：

//不使用查询表达式
            SampleData.Publishers.SelectMany(publisher => SampleData.Books.Select(
                book => new
                {
                    PublisherName = publisher.Name,
                    BookName = book.Title
                }
                ));

5.Linq实现左连接，写法如下

 var leftJoinSql = from student in db.Student
                              join book in db.Book on student.ID equals book.StudentID into temp
                              from tt in temp.DefaultIfEmpty()
                              select new
                              {
                                   sname= student.Name,
                                   bname = tt==null?"":tt.Name//这里主要第二个集合有可能为空。需要判断
                              };

6.Linq实现右连接，写法如下

 var rightJoinSql = from book in db.Book
                               join stu in db.Student on book.StudentID equals stu.ID into joinTemp
                               from tmp in joinTemp.DefaultIfEmpty()
                               select new { 
                               sname=tmp==null?"":tmp.Name,
                               bname=book.Name

                               };