LINQ to SQL 系列五 grouping having

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。

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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：

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SELECT [t0].[ClassID] AS [Key] FROM [dbo].[Student] AS [t0] GROUP BY [t0].[ClassID] -- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 4.0.30319.1   SELECT [t0].[StudentID], [t0].[Name], [t0].[Hometown], [t0].[Gender], [t0].[Birthday], [t0].[ClassID], [t0].[WeightInKg], [t0].[HeightInCm], [t0].[Desc] AS [Desc] FROM [dbo].[Student] AS [t0] WHERE @x1 = [t0].[ClassID] -- @x1: Input Int (Size = -1; Prec = 0; Scale = 0) [1] -- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 4.0.30319.1   SELECT [t0].[StudentID], [t0].[Name], [t0].[Hometown], [t0].[Gender], [t0].[Birthday], [t0].[ClassID], [t0].[WeightInKg], [t0].[HeightInCm], [t0].[Desc] AS [Desc] FROM [dbo].[Student] AS [t0] WHERE @x1 = [t0].[ClassID] -- @x1: Input Int (Size = -1; Prec = 0; Scale = 0) [2] -- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 4.0.30319.1   SELECT [t0].[StudentID], [t0].[Name], [t0].[Hometown], [t0].[Gender], [t0].[Birthday], [t0].[ClassID], [t0].[WeightInKg], [t0].[HeightInCm], [t0].[Desc] AS [Desc] FROM [dbo].[Student] AS [t0] WHERE @x1 = [t0].[ClassID] -- @x1: Input Int (Size = -1; Prec = 0; Scale = 0) [3] -- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 4.0.30319.1

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：

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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;               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。

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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：

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SELECT [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：

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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;               // 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：

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SELECT [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

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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

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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;               // 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：

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// 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

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SELECT [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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