在linq to sql中grouping很简单、很灵活，但是如果不注意则会中了Microsoft的糖衣炮弹。
本文使用的数据模型如下图：
1． 用linq to sql写一个最简单的group语句：
查询Students表并按ClassID分组，输出每个班级的学生数。下面的代码是糖衣版。
class="csharp keyword">static void Main(string[] args)
{
using (var writer = new StreamWriter(WatchSqlPath, false, Encoding.UTF8))
{
using (DbAppDataContext db = new DbAppDataContext())
{
//打印sql
db.Log = writer;
//最简单的group by ,按ClassID对Students进行分组
var query = from s in db.Students
group s by s.ClassID;
foreach (var item in query)
{
//输出班级编号和班级人数
Console.WriteLine("class id = {0} student count = {1}",item.Key,item.Count());
}
}
}
Console.ReadLine();
}
Linq to sql的写法很灵活，我们随时可以调用Count等一大堆方法，上例中我在输出时调用Count方法，导致了linq to sql真正执行的sql有一大坨，如下：
+ View Code这个不能怪linq to sql，我们这样写它就必须得那样执行；只能我们自己使用时处处小心。上例中正确的写法应该是：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29static void Main(string[] args)
{
using (var writer = new StreamWriter(WatchSqlPath, false, Encoding.UTF8))
{
using (DbAppDataContext db = new DbAppDataContext())
{
//打印sql
db.Log = writer;
//最简单的group by ,按ClassID对Students进行分组
//var query = from s in db.Students
// group s by s.ClassID;
var query = from s in db.Students
group s by s.ClassID into sg
select new {
ClassID = sg.Key,
Count = sg.Count()
};
foreach (var item in query)
{
//输出班级编号和班级人数
Console.WriteLine("class id = {0} student count = {1}", item.ClassID, item.Count);
}
}
}
Console.ReadLine();
}
这样执行时才是那个我们以前写t-sql时经常看到的那个group by语句。
2． 对分组聚合进行排序输出，我忘记了用let
请看下面代码，输出每个班级体重最大的同学，并要求最大体重得大于39，并按照体重大下，对分组结果进行排序。
static void Main(string[] args)
{
using (var writer = new StreamWriter(WatchSqlPath, false, Encoding.UTF8))
{
using (DbAppDataContext db = new DbAppDataContext())
{
//打印sql
db.Log = writer;
var query = from s in db.Students
group s by s.ClassID into gS
where gS.Max<Student>(s => s.WeightInKg) > 39
orderby gS.Max<Student>(s => s.WeightInKg) descending
select new
{
ClassID = gS.Key,
MaxWeight = gS.Max<Student>(s => s.WeightInKg)
};
foreach (var item in query)
{
Console.WriteLine("class id = {0} student max weight = {1}", item.ClassID, item.MaxWeight);
}
}
}
Console.ReadLine();
}
上例中，在query变量声明时我用了三次gS.Max<Student>(s => s.WeightInKg)，我们知道在t-sql中如果使用聚合值，必须得用表达式，在linq to sql中用会不会有问题呢，看下执行上述代码生成的sql吧：
1 2 3 4 5 6 7 8 9 10SELECT [t1].[ClassID], [t1].[value3] AS [MaxWeight]
FROM (
SELECT MAX([t0].[WeightInKg]) AS [value], MAX([t0].[WeightInKg]) AS [value2], MAX([t0].[WeightInKg]) AS [value3], [t0].[ClassID]
FROM [dbo].[Student] AS [t0]
GROUP BY [t0].[ClassID]
) AS [t1]
WHERE [t1].[value] > @p0
ORDER BY [t1].[value2] DESC
-- @p0: Input Float (Size = -1; Prec = 0; Scale = 0) [39]
-- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 4.0.30319.1
Linq to sql忠实的执行了我写的代码，我的query变量声明中用了三次Max，在t-sql的嵌套表中也有三个MAX对应，这个不是我想要的，我又错了，呼叫let，看看使用let之后linq to sql会怎么执行吧：
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27static void Main(string[] args)
{
using (var writer = new StreamWriter(WatchSqlPath, false, Encoding.UTF8))
{
using (DbAppDataContext db = new DbAppDataContext())
{
//打印sql
db.Log = writer;
// group by with having
var query = from s in db.Students
group s by s.ClassID into gS
let mw = gS.Max<Student>(s => s.WeightInKg)
where mw > 39
select new
{
ClassID = gS.Key,
MaxWeight = mw
};
foreach (var item in query)
{
Console.WriteLine("class id = {0} student max weight = {1}", item.ClassID, item.MaxWeight);
}
}
}
Console.ReadLine();
}
这次的sql语句比上一个好多了，但是还没有足够好：
1 2 3 4 5 6 7 8 9SELECT [t1].[ClassID], [t1].[value] AS [MaxWeight]
FROM (
SELECT MAX([t0].[WeightInKg]) AS [value], [t0].[ClassID]
FROM [dbo].[Student] AS [t0]
GROUP BY [t0].[ClassID]
) AS [t1]
WHERE [t1].[value] > @p0
-- @p0: Input Float (Size = -1; Prec = 0; Scale = 0) [39]
-- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 4.0.30319.1
还是多嵌套了一层没有必要的select。如何才能不嵌套这一层呢？如果你知道，请告诉我。
3． 使用linq to sql Group by多个字段：
这次我们先写一个标准的t-sql
SELECT ClassID,Hometown,count(*) Cn
FROM Student
GROUP BY ClassID,Hometown
ORDER BY ClassID,Hometown;
这个t-sql对Student表按照ClassID和Hometown两个字段进行分组，并输出每个班级中某个地方的学生数
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27static void Main(string[] args)
{
using (var writer = new StreamWriter(WatchSqlPath, false, Encoding.UTF8))
{
using (DbAppDataContext db = new DbAppDataContext())
{
//打印sql
db.Log = writer;
// group by with having
var query = from s in db.Students
group s by new { s.ClassID, s.Hometown } into gS
let cn = gS.Count<Student>()
select new
{
ClassID = gS.Key.ClassID,
Hometown = gS.Key.Hometown,
Count = cn
};
foreach (var item in query)
{
Console.WriteLine("class id = {0} hometown {1} student count = {2}", item.ClassID, item.Hometown,item.Count);
}
}
}
Console.ReadLine();
}
这一次linq to sql给了我一个惊喜，它用的t-sql和我开始写的一样。
4． 在3的基础上加一点点需求，要求分组后的结果按照count排序，query的声明代码如下：
1 2 3 4 5 6 7 8 9 10 11// group by with having
var query = from s in db.Students
group s by new { s.ClassID, s.Hometown } into gS
let cn = gS.Count<Student>()
orderby cn descending
select new
{
ClassID = gS.Key.ClassID,
Hometown = gS.Key.Hometown,
Count = cn
};
这次link to sql使用的sql语句多了一层没有必要的select嵌套
1 2 3 4 5 6 7 8SELECT [t1].[ClassID], [t1].[Hometown], [t1].[value] AS [Count]
FROM (
SELECT COUNT(*) AS [value], [t0].[ClassID], [t0].[Hometown]
FROM [dbo].[Student] AS [t0]
GROUP BY [t0].[ClassID], [t0].[Hometown]
) AS [t1]
ORDER BY [t1].[value] DESC
-- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 4.0.30319.1
看来使用linq to sql少多总要付出点代价的，linq to sql给了我们查询的方便、灵活，也给了我们性能的陷阱。
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