LINQ to SQL之内链接，左链接，右链接

示例如下：

var query = from tb0 in db.table_0
                        join tb1 in db.table_1 on table_0.关联键 equals table_1.关联键
                        select new { ... };

左链接或右链接，使用DefaultIfEmpty()语法，采用些语法前需要用到into语法

示例如下：
 var  query2 = from tb0 in db.table_0
                    join tb1 in db.table_1  on table_0.关联键 equals table_1.关联键
                    into all
                    from tb2 in all.DefaultIfEmpty()
                    select new { ... };