linq group
本篇介绍Linq的Group和Join操作,继续使用《Linq 学习(3) 语法结构》中介绍的数据源。
Group
Group是进行分组操作,同SQL中的Group By类似。
原型如下:
this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector)
它有几个重载,返回类型有两种:IEnumerable<IGrouping<TKey, TSource>> 和 IEnumerable<TResult>。
返回类型为:IEnumerable<IGrouping<TKey, TSource>>
示例:
返回按学号分组学生的成绩
group score by score.StudentID into scoreGroup
select scoreGroup;
scoreGroup为IGrouping<TKey, TSource>类型,返回结果为IEnumerable<IGrouping<TKey, TSource>>,既集合的集合,因此输出时需用双重循环。
IGrouping<TKey, TElement>接口定义为:
{
TKey Key { get; }
}
其中Key为分组依据的字段。
{
//输出分组依据的字段
Console.WriteLine("\nStudent ID:" + group.Key);
// 输出组内成员
foreach (var score in group)
{
Console.WriteLine(score);
}
}
// result:
// Student ID:1
// Student ID:1,Course ID:1,Score:78
// Student ID:1,Course ID:2,Score:60
// ...
// Student ID:2
// Student ID:2,Course ID:1,Score:59
// ...
等效的扩展方法调用实现为:
返回类型为:IEnumerable<TResult>
对分组结果进行一些包装,如包装为匿名类型。
返回按学号分组学生的成绩
group score by score.StudentID into scoreGroup
select new { StudentID = scoreGroup.Key, Group = scoreGroup };
匿名类型中Group为IGrouping<TKey, TSource>类型。
等效的扩展方法调用实现为:
(key, group) => new { StudentID = key, Group = group });
其他一些重载使用方法类似。
Join
连接操作。
this IEnumerable<TOuter> outer,
IEnumerable<TInner> inner,
Func<TOuter, TKey> outerKeySelector,
Func<TInner, TKey> innerKeySelector,
Func<TOuter, TInner, TResult> resultSelector)
从Join方法原型可以看出其使用方法。
内连接
选择左右两侧集合都含有相对应的元素。
示例:
查询学生的姓名、学科、成绩。
join student in DataSource.Students on score.StudentID equals student.StudentID
join course in DataSource.Courses on score.CourseID equals course.CourseID
select new { StudentName = student.Name, CourseName = course.CourseName, ScoreValue = score.Value };
// result
// { StudentName = Andy, CourseName = C Language, ScoreValue = 78 }
// { StudentName = Andy, CourseName = Biophysics, ScoreValue = 60 }
// ...
// { StudentName = Bill, CourseName = C Language, ScoreValue = 59 }
// { StudentName = Cindy, CourseName = Biophysics, ScoreValue = 60 }
// ...
等效的扩展方法调用实现为:
DataSource.Scores.Join(
DataSource.Students,
score => score.StudentID,
student => student.StudentID,
(score, student) => new { StudentName = student.StudentID, ScoreValue = score.Value, CourseID = score.CourseID })
.Join(DataSource.Courses,
scostu => scostu.CourseID,
course => course.CourseID,
(scostu, course) => new { StudentName = scostu.StudentName, CourseName = course.CourseName, ScoreValue = scostu.ScoreValue });
左外连接
当右侧的连接的右侧没有左侧对应的元素时,内连接会忽略左侧元素。要想保留左侧元素,可以使用做外连接。右侧被置为默认值,如:引用类型被置为空。
示例:
from student in DataSource.Students2
join score in DataSource.Scores on student.StudentID equals score.StudentID into Scores
from score in Scores.DefaultIfEmpty()
select new { student = student, score = score == default(Score) ? 0 : score.Value };
// result:
// { student = Student ID:5,Student Name:Erik, score = 78 }
// { student = Student ID:6,Student Name:Frank, score = 0 }
等效的扩展方法调用实现为:
var result =
DataSource.Students2.GroupJoin(
DataSource.Scores,
student => student.StudentID,
score => score.StudentID,
(student, Scores) => new { student = student, Scores = Scores })
.SelectMany(group => group.Scores.DefaultIfEmpty(),
(group, score) => new { student = group.student, score = (score == null) ? 0.0 : score.Value });
笛卡尔积
集合中的元素交错连接。
示例:统计学生课程成绩时的模板。
from course in DataSource.Courses
select new { StudentName = student.Name, CourseName = course.CourseName, ScoreValue = (double?)null };
// result:
// { StudentName = Andy, CourseName = C Language, ScoreValue = }
// { StudentName = Andy, CourseName = Biophysics, ScoreValue = }
// ...
// { StudentName = Bill, CourseName = C Language, ScoreValue = }
// ...
// { StudentName = Cindy, CourseName = Fundamentals of Compiling, ScoreValue = }
// ...
等效的扩展方法调用实现为:
var result = DataSource.Students.SelectMany(
student => DataSource.Courses
.Select(
course =>
new { StudentName = student.Name, CourseName = course.CourseName, ScoreValue = (double?)null }));
GroupJoin
连接分组。
方法原型为:
this IEnumerable<TOuter> outer,
IEnumerable<TInner> inner,
Func<TOuter, TKey> outerKeySelector,
Func<TInner, TKey> innerKeySelector,
Func<TOuter, IEnumerable<TInner>, TResult> resultSelector)
// result:
// Andy
// 1----78
// 2----60
// ...
// Bill
// 1----59
// ...
// Cindy
// 2----60
// ...
相当于组合了Group操作和Join操作。等效的操作如下:
(from student in DataSource.Students
join score in DataSource.Scores on student.StudentID equals score.StudentID
select new { StudentName = student.Name, CourseID = score.CourseID, Value = score.Value })
group item by item.StudentName into Group
select new { StudentName = Group.Key, Group = Group };
结束语
到现在,Linq与SQL语言等价的操作基本介绍完,组合这些操作能实现复杂的查询。
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