Detailed usage of the resultMap tag in mapper.xml of mybatis

There is a resultMap tag in mybatis, which is to map the collection of the results of the select query. Its main function is to associate the fields in the entity class with the fields in the database table. When the field in the entity class is the same as the field in the database table, you can ignore the association relationship in the resultMap tag. When the fields in the entity class are not the same as the fields in the database table, you need to associate the entity class fields with the database fields one by one in the resultMap tag, or turn on the hump rule to let it automatically convert.

To use resultMap, it is necessary to disable the camel case rule. If you don't want to change the entity class, you can use resultMap.

<?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">
<!--在mybatis中，映射文件中的namespace是用于绑定dao接口的，即面向接口编程。当你的namespace绑定接口后，可以不用写接口实现类
mybatis会通过绑定自动帮你找到对应要执行的sql语句-->
<!-- xmlns="http://mybatis.org/schema/mybatis-mapper" -->
<mapper namespace="com.example.page.mapper.UserMapper">
<!--在mybatis中有一个resultMap标签，它是为了映射select查询出来结果的集合，其主要作用是将实体类中的字段与数据表中的字段进行关联映射
注意
当实体类中的字段与数据库表中的字段相同时，可以将resultMap标签中的关联关系忽略不写
当实体类中的字段与数据库中的字段不相同时，就需要在resultMap标签中将实体类字段与数据库字段一一进行关联映射，或者开启驼峰规则，让它自动转换
-->
    <resultMap id="result" type="com.example.page.entity.User">
        <result property="userId" column="userId"/>
        <result property="userDate" column="userDate"/>
        <result property="userName" column="userName"/>
        <result property="userAddress" column="userAddress"/>
    </resultMap>
<!--    ListUser-->
    <select id="ListUser" resultMap="result">
        select * from user
    </select>
<!--    在Mapper.xml中可以通过#{}获取参数：
parameterType控制参数类型
#{}获取参数内容：
1：使用索引，从0开始#{0}表示第一个参数
2：也可以使用#{param1}第一个参数
3：如果参数时对象#{属性名}
4：如果参数是map写成#{key}
#{}和${}的区别：
#{}获取参数的内容支持，索引获取，param1获取指定位置参数，并且sql使用?占位符，${}字符串拼接不使用?,默认找${内容}内容的get/set方法
-->
<!--    findUserByName-->
    <select id="findUserByName" resultMap="result" parameterType="String">
        select * from user
        where userName like concat(concat('%',#{userName}),'%')
        order by userId desc
    </select>
<!--    queryPage-->
    <select id="queryPage" resultMap="result" parameterType="Integer">
        select * from user
        order by userId desc
        limit #{startRows},5
    </select>
<!--    select count(*)：查询所有行
select count(0)：忽略所有列，统计行数，与列无关-->
<!--    getRowCount-->
    <select id="getRowCount" resultType="Integer">
        select count(*) from user
    </select>
<!--    insertUser-->
    <insert id="insertUser" parameterType="com.example.page.entity.User">
        insert into user
        (userId,userDate,userName,userAddress)
        values
        (#{userId},#{userDate},#{userName},#{userAddress})
    </insert>
<!--    delete-->
    <delete id="delete" parameterType="int">
        delete from user where userId = #{userId}
    </delete>
<!--    update-->
    <update id="Update" parameterType="com.example.page.entity.User">
        update user
        set user.userDate=#{userDate},user.userName=#{userName},user.userAddress=#{userAddress}
        where user.userId=#{userId}
    </update>

</mapper>