@Select

package com.flying.entity;

public class Student {
    private Integer id;
    private String name;
    private Integer age;
    private String major;
    private Double score;

    // Getter 和 Setter 省略
    @Override
    public String toString() {
        return "Student{id=" + id + ", name='" + name + "', age=" + age +
               ", major='" + major + "', score=" + score + "}";
    }
}

package com.flying.mapper;

import com.flying.entity.Student;
import org.apache.ibatis.annotations.Select;
import java.util.List;
import java.util.Map;

public interface StudentMapper {

    /**
     * 查询所有学生
     */
    @Select("SELECT id, name, age, major, score FROM student")
    List<Student> selectAll();

    /**
     * 根据 ID 查询单个学生
     */
    @Select("SELECT id, name, age, major, score FROM student WHERE id = #{id}")
    Student selectById(Integer id);

    /**
     * 按专业模糊查询学生列表
     */
    @Select("SELECT id, name, age, major, score FROM student WHERE major LIKE CONCAT('%', #{keyword}, '%')")
    List<Student> selectByMajorLike(String keyword);

    /**
     * 统计各专业学生人数，返回 Map
     * key 为专业名称，value 为人数
     */
    @Select("SELECT major AS major, COUNT(*) AS cnt FROM student GROUP BY major")
    @MapKey("major")  // 注意：此处在返回 Map<String, Integer> 时实际由 SQL 别名控制
    List<Map<String, Object>> countByMajor();
}

package com.flying.test;

import com.flying.entity.Student;
import com.flying.mapper.StudentMapper;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.InputStream;
import java.util.List;
import java.util.Map;

public class SelectTest {
    public static void main(String[] args) throws Exception {
        InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(is);
        SqlSession session = factory.openSession();

        StudentMapper mapper = session.getMapper(StudentMapper.class);

        // 1. 查询所有学生
        System.out.println("=== 查询所有学生 ===");
        List<Student> all = mapper.selectAll();
        all.forEach(System.out::println);

        // 2. 根据 ID 查询
        System.out.println("\n=== 根据 ID 查询 ===");
        Student stu = mapper.selectById(1);
        System.out.println(stu);

        // 3. 模糊查询专业包含'软件'的学生
        System.out.println("\n=== 模糊查询 ===");
        List<Student> software = mapper.selectByMajorLike("软件");
        software.forEach(System.out::println);

        // 4. 按专业统计人数
        System.out.println("\n=== 按专业统计 ===");
        List<Map<String, Object>> stats = mapper.countByMajor();
        stats.forEach(System.out::println);

        session.close();
    }
}