siverlight linq語句和lambda表達式

• 這裏介紹Linq使用Group By和Count得到每個CategoryID中產品的數量，Linq使用Group By和Count得到每個CategoryID中斷貨產品的數量等方面。

學經常會遇到Linq使用Group By問題，這裏將介紹Linq使用Group By問題的解決方法。

1.計數

1. var q =
2. from p in db.Products
3. group p by p.CategoryID into g
4. select new {
5. g.Key,
6. NumProducts = g.Count()
7. };

語句描述：Linq使用Group By和Count得到每個CategoryID中產品的數量。

說明：先按CategoryID歸類，取出CategoryID值和各個分類產品的數量。

2.帶條件計數

1. var q =
2. from p in db.Products
3. group p by p.CategoryID into g
4. select new {
5. g.Key,
6. NumProducts = g.Count(p => p.Discontinued)
7. };

語句描述：Linq使用Group By和Count得到每個CategoryID中斷貨產品的數量。

說明：先按CategoryID歸類，取出CategoryID值和各個分類產品的斷貨數量。 Count函數裏，使用了Lambda表達式，Lambda表達式中的p，代表這個組裏的一個元素或對象，即某一個產品。

3.Where限制

1. var q =
2. from p in db.Products
3. group p by p.CategoryID into g
4. where g.Count() >= 10
5. select new {
6. g.Key,
7. ProductCount = g.Count()
8. };

語句描述：根據產品的―ID分組，查詢產品數量大於10的ID和產品數量。這個示例在Group By子句後使用Where子句查找所有至少有10種產品的類別。

說明：在翻譯成SQL語句時，在最外層嵌套了Where條件。

4.多列(Multiple Columns)

1. var categories =
2. from p in db.Products
3. group p by new
4. {
5. p.CategoryID,
6. p.SupplierID
7. }
8. into g
9. select new
10. {
11. g.Key,
12. g
13. };

語句描述：Linq使用Group By按CategoryID和SupplierID將產品分組。

說明：既按產品的分類，又按供應商分類。在by後面，new出來一個匿名類。這裏，Key其實質是一個類的對象，Key包含兩個Property：CategoryID、SupplierID。用g.Key.CategoryID可以遍歷CategoryID的值。

5.表達式(Expression)

1. var categories =
2. from p in db.Products
3. group p by new { Criterion = p.UnitPrice > 10 } into g
4. select g;

語句描述：Linq使用Group By返回兩個產品序列。第一個序列包含單價大於10的產品。第二個序列包含單價小於或等於10的產品。

說明：按產品單價是否大於10分類。其結果分爲兩類，大於的是一類，小於及等於爲另一類。

 

6.舉例：

var lst = from p in DC.FWorkBills group p by p.WorkState into g select new {工程狀態=g.Key , 數量=g.Count()}; dataGridGarb2.ItemsSource = lst.ToList(); dataGridGarb2.DataContext = lst.ToList();

7.對比

sql語句-linq語言-lambda表達式對照

1、 查詢Student表中的所有記錄的Sname、Ssex和Class列。
select sname,ssex,class from student
Linq:
    from s in Students
    select new {
        s.SNAME,
        s.SSEX,
        s.CLASS
    }
Lambda:
    Students.Select( s => new {
        SNAME = s.SNAME,SSEX = s.SSEX,CLASS = s.CLASS
    })

2、 查詢教師所有的單位即不重複的Depart列。
select distinct depart from teacher
Linq:
    from t in Teachers.Distinct()
    select t.DEPART
Lambda:
    Teachers.Distinct().Select( t => t.DEPART)

 

3、 查詢Student表的所有記錄。
select * from student
Linq:
    from s in Students
    select s
Lambda:
    Students.Select( s => s)

4、 查詢Score表中成績在60到80之間的所有記錄。
select * from score where degree between 60 and 80
Linq:
    from s in Scores
    where s.DEGREE >= 60 && s.DEGREE < 80
    select s
Lambda:
    Scores.Where(
        s => (
                s.DEGREE >= 60 && s.DEGREE < 80
             )
    )

 

5、 查詢Score表中成績爲85，86或88的記錄。
select * from score where degree in (85,86,88)
Linq:
In
    from s in Scores
    where (
            new decimal[]{85,86,88}
          ).Contains(s.DEGREE)
    select s
Lambda:
    Scores.Where( s => new Decimal[] {85,86,88}.Contains(s.DEGREE))
Not in
    from s in Scores
    where !(
            new decimal[]{85,86,88}
          ).Contains(s.DEGREE)
    select s
Lambda:
    Scores.Where( s => !(new Decimal[]{85,86,88}.Contains(s.DEGREE)))

    Any()應用：雙表進行Any時，必須是主鍵爲(String)
    CustomerDemographics CustomerTypeID（String）
    CustomerCustomerDemos (CustomerID CustomerTypeID) (String)
    一個主鍵與二個主建進行Any(或者是一對一關鍵進行Any)
    不可，以二個主鍵於與一個主鍵進行Any
   
    from e in CustomerDemographics
    where !e.CustomerCustomerDemos.Any()
    select e
   
    from c in Categories
    where !c.Products.Any()
    select c

 

6、 查詢Student表中"95031"班或性別爲"女"的同學記錄。
select * from student where class ='95031' or ssex= N'女'
Linq:
    from s in Students
    where s.CLASS == "95031"
       || s.CLASS == "女"
    select s
Lambda:
    Students.Where(s => ( s.CLASS == "95031" || s.CLASS == "女"))

7、 以Class降序查詢Student表的所有記錄。
select * from student order by Class DESC
Linq:
    from s in Students
    orderby s.CLASS descending
    select s
Lambda:
    Students.OrderByDescending(s => s.CLASS)

 

8、 以Cno升序、Degree降序查詢Score表的所有記錄。
select * from score order by Cno ASC,Degree DESC
Linq:(這裏Cno ASC在linq中要寫在最外面)
    from s in Scores
    orderby s.DEGREE descending
    orderby s.CNO ascending
    select s
Lambda:
    Scores.OrderByDescending( s => s.DEGREE)
          .OrderBy( s => s.CNO)

9、 查詢"95031"班的學生人數。
select count(*) from student where class = '95031'
Linq:
    (    from s in Students
        where s.CLASS == "95031"
        select s
    ).Count()
Lambda:
    Students.Where( s => s.CLASS == "95031" )
                .Select( s => s)
                    .Count()

 

10、查詢Score表中的最高分的學生學號和課程號。
select distinct s.Sno,c.Cno from student as s,course as c ,score as sc
where s.sno=(select sno from score where degree = (select max(degree) from score))
and c.cno = (select cno from score where degree = (select max(degree) from score))
Linq:
    (
        from s in Students
        from c in Courses
        from sc in Scores
        let maxDegree = (from sss in Scores
                        select sss.DEGREE
                        ).Max()
        let sno = (from ss in Scores
                where ss.DEGREE == maxDegree
                select ss.SNO).Single().ToString()
        let cno = (from ssss in Scores
                where ssss.DEGREE == maxDegree
                select ssss.CNO).Single().ToString()
        where s.SNO == sno && c.CNO == cno
        select new {
            s.SNO,
            c.CNO
        }
    ).Distinct()
操作時問題？執行時報錯： where s.SNO == sno（這行報出來的） 運算符"=="無法應用於"string"和"System.Linq.IQueryable<string>"類型的操作數
解決：
原：let sno = (from ss in Scores
                where ss.DEGREE == maxDegree
                select ss.SNO).ToString()
Queryable().Single()返回序列的唯一元素；如果該序列並非恰好包含一個元素，則會引發異常。
解：let sno = (from ss in Scores
                where ss.DEGREE == maxDegree
                select ss.SNO).Single().ToString()
 

11、查詢'3-105'號課程的平均分。
select avg(degree) from score where cno = '3-105'
Linq:
    (
        from s in Scores
        where s.CNO == "3-105"
        select s.DEGREE
    ).Average()
Lambda:
    Scores.Where( s => s.CNO == "3-105")
            .Select( s => s.DEGREE)
                .Average()

12、查詢Score表中至少有5名學生選修的並以3開頭的課程的平均分數。
select avg(degree) from score where cno like '3%' group by Cno having count(*)>=5
Linq:
        from s in Scores
        where s.CNO.StartsWith("3")
        group s by s.CNO
        into cc
        where cc.Count() >= 5
        select cc.Average( c => c.DEGREE)
Lambda:
    Scores.Where( s => s.CNO.StartsWith("3") )
            .GroupBy( s => s.CNO )
              .Where( cc => ( cc.Count() >= 5) )
                .Select( cc => cc.Average( c => c.DEGREE) )
Linq: SqlMethod
like也可以這樣寫:
    s.CNO.StartsWith("3") or SqlMethods.Like(s.CNO,"%3")

 

13、查詢最低分大於70，最高分小於90的Sno列。
select sno from score group by sno having min(degree) > 70 and max(degree) < 90
Linq:
    from s in Scores
    group s by s.SNO
    into ss
    where ss.Min(cc => cc.DEGREE) > 70 && ss.Max( cc => cc.DEGREE) < 90
    select new
    {
        sno = ss.Key
    }
Lambda:
    Scores.GroupBy (s => s.SNO)
               .Where (ss => ((ss.Min (cc => cc.DEGREE) > 70) && (ss.Max (cc => cc.DEGREE) < 90)))
                   .Select ( ss => new {
                                        sno = ss.Key
                                     })

 

14、查詢所有學生的Sname、Cno和Degree列。
select s.sname,sc.cno,sc.degree from student as s,score as sc where s.sno = sc.sno
Linq:
    from s in Students
    join sc in Scores
    on s.SNO equals sc.SNO
    select new
    {
        s.SNAME,
        sc.CNO,
        sc.DEGREE
    }
Lambda:
    Students.Join(Scores, s => s.SNO,
                          sc => sc.SNO,
                          (s,sc) => new{
                                            SNAME = s.SNAME,
                                            CNO = sc.CNO,
                                            DEGREE = sc.DEGREE
                                          })

 

15、查詢所有學生的Sno、Cname和Degree列。
select sc.sno,c.cname,sc.degree from course as c,score as sc where c.cno = sc.cno
Linq:
    from c in Courses
    join sc in Scores
    on c.CNO equals sc.CNO
    select new
    {
        sc.SNO,c.CNAME,sc.DEGREE
    }
Lambda:
    Courses.Join ( Scores, c => c.CNO,
                             sc => sc.CNO,
                             (c, sc) => new 
                                        {
                                            SNO = sc.SNO,
                                            CNAME = c.CNAME,
                                            DEGREE = sc.DEGREE
                                        })

 

16、查詢所有學生的Sname、Cname和Degree列。
select s.sname,c.cname,sc.degree from student as s,course as c,score as sc where s.sno = sc.sno and c.cno = sc.cno
Linq:
    from s in Students
    from c in Courses
    from sc in Scores
    where s.SNO == sc.SNO && c.CNO == sc.CNO
    select new { s.SNAME,c.CNAME,sc.DEGREE }