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。
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:
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:
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。
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:
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:
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:
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
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
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:
// 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
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。
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