myBatis的基础查询

单条或多条查询

根据id查询单条数据，查询所有数据的列表集合，查询所有数据的条目，查询出单条数据返回值为map,查询多条数据返回值为列表，查询多条数据返回值为map

步骤

创建SelectMapper接口和SelectMapper.xml映射文件和test类
SelectMapper

package com.wjw.mybatis.mapper;

import com.wjw.mybatis.pojo.User;
import org.apache.ibatis.annotations.MapKey;
import org.apache.ibatis.annotations.Param;

import java.util.List;
import java.util.Map;

/**
 * @Author wjwdive
 * @Date 2023/2/25 22:39
 * @Description:
 */
public interface SelectMapper {

    /**
     * 根据Id查询用户信息
     * @param id
     * @return
     */
    User getUserById(@Param("id") Integer id);

    /**
     * 查询所有用户信息
     * @return
     */
    List<User> getAllUser();

    /**
     * 查询用户的总数量
     * @return
     */
    Integer getCount();

    /**
     * 根据id查询用户信息为map集合
     * @param id
     * @return
     */
    Map<String, Object> getUserByIdToMap(@Param("id") Integer id);

    /**
     * 查询所有用户信息返回值为   List<Map<String, Object>
     * @return
     */
    List<Map<String, Object> >getAllUserToMap();

    /**
     * 查询所有用户信息，返回值为 map   要用@Mapkey("id") 注解
     * @return
     */
    @MapKey("id")
    Map<String, Object> getAllUserToMap2();
}

SelectMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.wjw.mybatis.mapper.SelectMapper">
<!--    User getUserById(@Param("id") Integer id);-->
    <select id="getUserById" resultType="User">
        select * from t_user where id = #{id};
    </select>

<!--    List<User> getAllUser();-->
    <select id="getAllUser" resultType="User">
        select * from t_user;
    </select>

<!--    Integer getCount();-->
    <select id="getCount" resultType="int">
        select count(*) from t_user;
    </select>

<!--    Map<String, Object> getUserByIdToMap(@Param("id") Integer id);-->
    <select id="getUserByIdToMap" resultType="map">
        select * from t_user where id = #{id};
    </select>

<!--    List<Map<String, Object> >getAllUserToMap();-->
    <select id="getAllUserToMap" resultType="User">
        select * from t_user;
    </select>

    <!--    List<Map<String, Object> >getAllUserToMap2();-->
    <select id="getAllUserToMap2" resultType="User">
        select * from t_user;
    </select>

</mapper>

test类

package com.wjw.mybatis.test;

import com.wjw.mybatis.mapper.SelectMapper;
import com.wjw.mybatis.pojo.User;
import com.wjw.mybatis.utils.SqlSessionUtil;
import org.apache.ibatis.annotations.Select;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import java.util.List;
import java.util.Map;

/**
 * @Author wjwdive
 * @Date 2023/2/25 23:22
 * @Description:
 */
public class SelectMapperTest {
    @Test
    public void testSelectUserById() {
        SqlSession sqlSession = SqlSessionUtil.getSqlSession();
        SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
        User user = mapper.getUserById(2);
        System.out.println("user: " + user );
    }

    @Test
    public void testGetAllUsers() {
        SqlSession sqlSession = SqlSessionUtil.getSqlSession();
        SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
        List<User> list = mapper.getAllUser();
        System.out.println("users: ");
        list.forEach(System.out::println);
//        System.out.println("users: " + list );
    }

    @Test
    public void testGetCount() {
        SqlSession sqlSession = SqlSessionUtil.getSqlSession();
        SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
        Integer totalUsers = mapper.getCount();
        System.out.println("totalUser counts: " + totalUsers );
    }

    @Test
    public void testGetUserByIdToMap() {
        SqlSession sqlSession = SqlSessionUtil.getSqlSession();
        SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
        Map<String, Object> map = mapper.getUserByIdToMap(3);
        System.out.println("user map: " + map);
    }

    @Test
    public void getAllUserToMap() {
        SqlSession sqlSession = SqlSessionUtil.getSqlSession();
        SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
        List<Map<String, Object> > list = mapper.getAllUserToMap();
        System.out.println("all user list of map: " + list);
    }

    @Test
    public void getAllUserToMap2() {
        SqlSession sqlSession = SqlSessionUtil.getSqlSession();
        SelectMapper mapper = sqlSession.getMapper(SelectMapper.class);
        Map<String, Object> map = mapper.getAllUserToMap2();
        System.out.println("all users of map: " + map);
    }


}