public void onCreate(sqliteDatabase db) 
	{
		// TODO Auto-generated method stub
		String classessql = "CREATE TABLE classes(class_id varchar(10) primary key," +
        		"class_name varchar(20))";
		
		String studentssql = "CREATE TABLE students(student_id varchar(10) primary key," +
        		"student_name varchar(20),score varchar(4),class_id varchar(10)," +
        		"foreign key (class_id) references classes(class_id) " +
        		"on delete cascade on update cascade )";
		db.execsql(classessql);
		Log.d("my","create table classes:"+classessql);
		db.execsql(studentssql);
		Log.d("my","create table students:"+studentssql);
		
		
	}

on delete cascade on update cascade

失效了。

经查资料知道：sqlite在3.6.19版本中才开始支持外键约束，但是为了兼容以前的程序，默认并没有启用该功能，如果要启用该功能每次都要需要使用如下语句：PRAGMA foreign_keys = ON来打开。

也就是说，在执行删除一个班级的语句的时候需要执行db.execsql("PRAGMA foreign_keys=ON")

/**
	 * 删除一个班级
	 * 同时会删除students中该班级的学生
	 * @param class_id
	 */
	public void deleteClass(String class_id)
	{
	    sqliteDatabase localsqliteDatabase = this.dbhelper.getWritableDatabase();
	    //设置了级联删除和级联更新
	    //在执行有级联关系的语句的时候必须先设置“PRAGMA foreign_keys=ON”
	    //否则级联关系默认失效
	    localsqliteDatabase.execsql("PRAGMA foreign_keys=ON");
	    Object[] arrayOfObject = new Object[1];
	    arrayOfObject[0] =class_id;
	    localsqliteDatabase.execsql("delete from classes where class_id=?",arrayOfObject);
	    localsqliteDatabase.close();
	}