resultMap 映射

1. sql的重用：定义一个sql片段，可在任何SQL语句中重用该片段。

<sql id="personColumns"> name, sex, updateTime</sql>
<select id="selectPerson" parameterType="int" resultType="hashmap">
    select id, <include refid="personColumns"/> from person where id =#{id};
</select>

2. javabean别名：不用每次写包路径

<!-- In Config XML file，定义 -->
<typeAlias type=”com.someapp.model.User” alias=”User”/>
<!-- In SQL Mapping XML file，使用 -->
<select id=”selectUsers” parameterType=”int” resultType=”User”>
    select id, username, hashedPassword from some_table where id = #{id}
</select>

3. 表与实体列名不匹配的解决

a) SQL的别名

<select id=”selectUsers” parameterType=”int” resultType=”User”>
select user_id as "id", user_name as userName, hashed_password as hashedPassword from some_table where id = #{id}
</select>

b)定义外部的resultMap

<resultMap id="userResult" type="User">
    <id property="id" column="_id" />
    <result property="name" column="_name" />
    <result property="password" column="_password" />
</resultMap>

<select id="selectUser" parameterType="int" resultMap="userResult">
    select _id, _name, _password from _user where _id =#{id};
</select>