写成代码，含有两三项功能即可

以下是用C语言实现的一个简单的在线学习网站后端示例代码，包括了用户登录、注册和课程管理三个功能：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <mysql/mysql.h>

// 用户结构体
typedef struct {
    int id;
    char username[50];
    char password[50];
} User;

// 课程结构体
typedef struct {
    int id;
    char name[50];
    char description[100];
    int user_id;
} Course;

// 连接数据库
MYSQL *connect_db() {
    MYSQL *conn = mysql_init(NULL);
    if (!mysql_real_connect(conn, "localhost", "root", "password", "learning", 0, NULL, 0)) {
        printf("Error connecting to database: %s\n", mysql_error(conn));
        exit(1);
    }
    return conn;
}

// 用户登录
User *login(char *username, char *password) {
    MYSQL *conn = connect_db();
    MYSQL_RES *res;
    MYSQL_ROW row;
    User *user = NULL;
    char query[200];
    sprintf(query, "SELECT * FROM users WHERE username='%s' AND password='%s'", username, password);
    if (mysql_query(conn, query)) {
        printf("Error executing query: %s\n", mysql_error(conn));
        exit(1);
    }
    res = mysql_use_result(conn);
    if (res) {
        if ((row = mysql_fetch_row(res))) {
            user = (User *)malloc(sizeof(User));
            user->id = atoi(row[0]);
            strcpy(user->username, row[1]);
            strcpy(user->password, row[2]);
        }
        mysql_free_result(res);
    }
    mysql_close(conn);
    return user;
}

// 用户注册
User *register_user(char *username, char *password) {
    MYSQL *conn = connect_db();
    MYSQL_RES *res;
    MYSQL_ROW row;
    User *user = NULL;
    char query[200];
    sprintf(query, "INSERT INTO users (username, password) VALUES ('%s', '%s')", username, password);
    if (mysql_query(conn, query)) {
        printf("Error executing query: %s\n", mysql_error(conn));
        exit(1);
    }
    sprintf(query, "SELECT * FROM users WHERE username='%s' AND password='%s'", username, password);
    if (mysql_query(conn, query)) {
        printf("Error executing query: %s\n", mysql_error(conn));
        exit(1);
    }
    res = mysql_use_result(conn);
    if (res) {
        if ((row = mysql_fetch_row(res))) {
            user = (User *)malloc(sizeof(User));
            user->id = atoi(row[0]);
            strcpy(user->username, row[1]);
            strcpy(user->password, row[2]);
        }
        mysql_free_result(res);
    }
    mysql_close(conn);
    return user;
}

// 添加课程
void add_course(char *name, char *description, int user_id) {
    MYSQL *conn = connect_db();
    char query[200];
    sprintf(query, "INSERT INTO courses (name, description, user_id) VALUES ('%s', '%s', %d)", name, description, user_id);
    if (mysql_query(conn, query)) {
        printf("Error executing query: %s\n", mysql_error(conn));
        exit(1);
    }
    mysql_close(conn);
}

// 删除课程
void delete_course(int course_id, int user_id) {
    MYSQL *conn = connect_db();
    char query[200];
    sprintf(query, "DELETE FROM courses WHERE id=%d AND user_id=%d", course_id, user_id);
    if (mysql_query(conn, query)) {
        printf("Error executing query: %s\n", mysql_error(conn));
        exit(1);
    }
    mysql_close(conn);
}

// 获取用户的所有课程
Course **get_courses(int user_id) {
    MYSQL *conn = connect_db();
    MYSQL_RES *res;
    MYSQL_ROW row;
    Course **courses = NULL;
    int num_courses = 0;
    char query[200];
    sprintf(query, "SELECT * FROM courses WHERE user_id=%d", user_id);
    if (mysql_query(conn, query)) {
        printf("Error executing query: %s\n", mysql_error(conn));
        exit(1);
    }
    res = mysql_use_result(conn);
    if (res) {
        while ((row = mysql_fetch_row(res))) {
            courses = (Course **)realloc(courses, sizeof(Course *) * (num_courses + 1));
            courses[num_courses] = (Course *)malloc(sizeof(Course));
            courses[num_courses]->id = atoi(row[0]);
            strcpy(courses[num_courses]->name, row[1]);
            strcpy(courses[num_courses]->description, row[2]);
            courses[num_courses]->user_id = atoi(row[3]);
            num_courses++;
        }
        mysql_free_result(res);
    }
    mysql_close(conn);
    return courses;
}

int main() {
    // 登录
    User *user = login("john", "123456");
    if (user) {
        printf("User %s logged in successfully.\n", user->username);
    } else {
        printf("Invalid username or password.\n");
    }

    // 注册
    user = register_user("mary", "abcdef");
    if (user) {
        printf("User %s registered successfully.\n", user->username);
    } else {
        printf("Registration failed.\n");
    }

    // 添加课程
    add_course("Math", "Learn math from basic arithmetic to calculus.", 1);
    add_course("History", "Explore the history of human civilization.", 2);

    // 删除课程
    delete_course(2, 2);

    // 获取用户的所有课程
    Course **courses = get_courses(1);
    printf("User 1 has %d courses:\n", sizeof(courses));
    for (int i = 0; i < sizeof(courses); i++) {
        printf("%d. %s: %s\n", courses[i]->id, courses[i]->name, courses[i]->description);
    }
    return 0;
}

以上代码仅为示例，实际实现时需要根据具体需求进行修改和完善。