Mybatis XML 映射文件详解

TIP

XML 映射文件是 MyBatis 的核心，定义了 SQL 语句、参数映射和结果映射。

基本结构

<?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.example.mapper.UserMapper">
    <!-- SQL 定义 -->
</mapper>

结果映射

<!-- 基础结果映射 -->
<resultMap id="userMap" type="com.example.entity.User">
    <id column="id" property="id"/>
    <result column="user_name" property="userName"/>
    <result column="age" property="age"/>
    <result column="email" property="email"/>
    <result column="created_at" property="createdAt"/>
</resultMap>

<!-- 关联查询映射 -->
<resultMap id="userOrderMap" type="com.example.entity.User" extends="userMap">
    <collection property="orders" ofType="com.example.entity.Order">
        <id column="order_id" property="id"/>
        <result column="order_no" property="orderNo"/>
        <result column="amount" property="amount"/>
    </collection>
</resultMap>

<select id="selectUserWithOrders" resultMap="userOrderMap">
    SELECT u.*, o.id as order_id, o.order_no, o.amount
    FROM user u
    LEFT JOIN `order` o ON u.id = o.user_id
    WHERE u.id = #{id}
</select>

一对一和一对多

<!-- 一对一关联 -->
<association property="profile" column="id"
    select="com.example.mapper.ProfileMapper.selectByUserId"/>

<!-- 一对多关联 -->
<collection property="orders" column="id"
    select="com.example.mapper.OrderMapper.selectByUserId"/>

参数传递

<!-- 单个参数 -->
<select id="selectById" resultType="User">
    SELECT * FROM user WHERE id = #{id}
</select>

<!-- 多个参数 -->
<select id="selectByNameAndAge" resultType="User">
    SELECT * FROM user WHERE name = #{name} AND age = #{age}
</select>

<!-- 使用 @Param 注解 -->
<select id="selectByCondition" resultType="User">
    SELECT * FROM user
    WHERE name = #{name}
    <if test="age != null">
        AND age = #{age}
    </if>
</select>

主键回填

<!-- 插入后返回自增主键 -->
<insert id="insert" useGeneratedKeys="true" keyProperty="id">
    INSERT INTO user(name, age, email) VALUES(#{name}, #{age}, #{email})
</insert>