LinQ学习记录（一）

1.from 子句

 		int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
 		
        var lowNums = from num in numbers
            where num < 5
            select num;
            
        foreach (int i in lowNums)
        {
            Console.Write(i + " ");
        }

查询语句会使用from进行开头

2.复合 from 子句

var scoreQuery = from student in students
     from score in student.Scores
     where score > 90
     select new { Last = student.LastName, score };

当内部数据源本身是序列（可进行迭代）的时候，便可以使用复合from，对序列内的数据进行调用
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var joinQuery2 =
	from lower in lowerCase
	where lower != 'x'
	from upper in upperCase
	select new { lower, upper };

使用from访问多个数据源

3. where 子句

var queryLowNums =
	from num in numbers
	where num < 5
	select num;

where 子句用在查询表达式中，用于指定一个返回的标准
其可以出现在除了第一句和最后一句之外的所有位置
如果删除where，则会返回所有的数据
可以使用&&、||来指定多个不同的标准（谓词）
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 var queryEvenNums =
            from num in numbers
            where IsEven(num)
            select num;
 static bool IsEven(int i)
    {
        return i % 2 == 0;
    }

甚至可以指定返回值为bool的方法，来作为挑选的“标准”

4. select 子句

IEnumerable<int> queryHighScores =
	from score in Scores
	where score > 80
	select score;

select 子句指定在执行查询时产生的值的类型
查询表达式必须以 select 子句或 group 子句结尾
5. group 子句

var studentQuery1 =
    from student in students
    group student by student.Last[0];

使用group by对数据进行分组，分组之后的结果需要嵌套foreach循环进行取出
——————————————————————————

var wordGroups2 =
	from w in words2
	group w by w[0] into grps
	where (grps.Key == 'a' || grps.Key == 'e' || grps.Key == 'i'
	|| grps.Key == 'o' || grps.Key == 'u')
	select grps;

可以使用into关键字进行逻辑的附加，此时要用其他关键字进行结尾
by之后的键类型可以按照字符串、布尔值、复合键等条件进行分组
范例如下：

var studentQuery3 =
    from student in students
    group student by student.Last;
    //字符串
var booleanGroupQuery =
	from student in students
	group student by student.Scores.Average() >= 80; 
	//布尔值
var studentQuery =
	from student in students
	let avg = (int)student.Scores.Average()
	group student by (avg / 10) into g
	orderby g.Key
	select g;
	//数值范围
group person by new {name = person.surname, city = person.city};
	//复合键

6. into标识符
其作用是将 group、join 或 select 子句的结果，存储至新标识符，代表着一种“延续”

var wordGroups1 =
	from w in words
	group w by w[0] into fruitGroup
	where fruitGroup.Count() >= 2
	select new { FirstLetter = fruitGroup.Key, Words = fruitGroup.Count() };

7. orderby 子句
可以使返回值按照升序或者降序进行排序
如果是字符串，将会以A开始

IEnumerable<string> sortAscendingQuery =
	from fruit in fruits
	orderby fruit ascending
	select fruit;

IEnumerable<string> sortDescendingQuery =
	from w in fruits
	orderby w descending
	select w;

ascending为默认关键字，可以进行省略，代表正序进行排列‘
descending关键字代表以倒序进行排列
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IEnumerable<Student> sortedStudents =
	from student in students
	orderby student.Last ascending, student.First ascending
	select student;

可以填入多个数据，代表进行主要排列和次要排列