如何获取不同的值节点XML

<?xml version="1.0" encoding="UTF-8"?>
    <Students> -<Student Id="001">
        <Name>Peter</Name>
        <LastName>Kohen</LastName> -<Courses> -<Course Id="01">
                <Name>C#</Name>
            </Course> -<Course Id="02">
                <Name>Java</Name>
            </Course>
        </Courses>
    </Student> -<Student Id="002">
        <Name>Nick</Name>
        <LastName>Nikes</LastName> -<Courses> -<Course Id="02">
                <Name>Java</Name>
            </Course> -<Course Id="03">
                <Name>Oracle</Name>
            </Course>
        </Courses>
    </Student> -<Student Id="003">
        <Name>Rafi</Name>
        <LastName>rafifa</LastName> -<Courses> -<Course Id="02">
                <Name>Java</Name>
            </Course> -<Course Id="03">
                <Name>Oracle</Name>
            </Course>
        </Courses>
    </Student> -<Student Id="004">
        <Name>Yosi</Name>
        <LastName>Koen</LastName> -<Courses> -<Course Id="04">
                <Name>SQL</Name>
            </Course> -<Course Id="03">
                <Name>Oracle</Name>
            </Course>
        </Courses>
    </Student>
</Students>

我需要知道两件事：

>所有课程
>每个课程中有多少学生在学习.

//Course[not(preceding::Course/@Id = @Id)]