LINQ to SQL系列四 使用inner join，outer join
本文中涉及的表数据模型请参考上一篇文章
先看一个最简单的inner join，在读取Student表时inner join Class表取的对应的Class信息：
static void Main(string[] args)
{
using (var writer = new StreamWriter(WatchSqlPath, false, Encoding.UTF8))
{
using (DbAppDataContext db = new DbAppDataContext())
{
db.Log = writer;
//inner join
var query = from s in db.Students
join c in db.Classes on s.ClassID equals c.ClassID
where c.ClassID == 1
select new
{
ClassID = s.ClassID,
ClassName = c.ClassName,
Student = new
{
Name = s.Name,
ID = s.StudentID
}
};
foreach (var item in query)
{
Console.WriteLine("{0} {1} {2}", item.ClassID, item.ClassName, item.Student.Name);
}
}
}
Console.ReadLine();
}
特别简单，就不贴sql了，继续看outer join：
static void Main(string[] args)
{
using (var writer = new StreamWriter(WatchSqlPath, false, Encoding.UTF8))
{
using (DbAppDataContext db = new DbAppDataContext())
{
db.Log = writer;
//left outer join
var query = from s in db.Students
join c in db.Classes on s.ClassID equals c.ClassID into gc
from gci in gc.DefaultIfEmpty()
select new
{
ClassID = s.ClassID,
ClassName = gci.ClassName,
Student = new
{
Name = s.Name,
ID = s.StudentID
}
};
foreach (var item in query)
{
Console.WriteLine("{0} {1} {2}", item.ClassID, item.ClassName, item.Student.Name);
}
}
}
Console.ReadLine();
}
Outer join时必须将join后的表into到一个新的变量gc中，然后要用gc.DefaultIfEmpty()表示外连接。
linq to sql相关随笔：
1． 从CUD开始，如何使用LINQ to SQL插入、修改、删除数据
3． 查询 延迟加载与立即加载，使用LoadWith和AssociateWith
4． 查询 inner join，left outer join
