mybatis的association集合嵌套

学了一下mybatis的查询返回值的集合嵌套，先查了查官网：

这个返回集合有什么用呢？举个例子三张表

hr_job_department

hr_job_position

第三张表里在表示部门和职位的时候只用了上面两张表的主键

但是查询的时候，希望表示下面这样的结果

所以返回值是不止一个对象，这样就用到了集合嵌套

<resultMap id="userInfoMap" type="com.advancedc.hrsys.entity.UserInfo">
		<id column="id" property="id" />
		<result column="name" property="name" />
		<result column="gender" property="gender" />
		<result column="id_card" property="idCard" />
		<result column="is_married" property="isMarried" />
		<result column="phone" property="phone" />
		<result column="priority" property="priority" />
		<result column="entry_time" property="entryTime" />
		<result column="full_time" property="fullTime" />
		<result column="created_time" property="createdTime" />
		<result column="edited_time" property="editedTime" />
		<association property="jobDepartment" column="id"
			javaType="com.advancedc.hrsys.entity.JobDepartment">
			<id column="jdid" property="id" />
			<result column="jdname" property="name" />
		</association>
		<association property="jobPosition" column="id"
			javaType="com.advancedc.hrsys.entity.JobPosition">
			<id column="jpid" property="id" />
			<result column="jpname" property="name" />
		</association>
	</resultMap>

只需要知道：

（1）column表示数据库字段

（2）property表示Java里的值

而且我这里的主键都是id所以会出现重名的情况，在SQL语句里，查询时就要赋予别名才能加以区分，返回结果resultMap就如上图所示 

<select id="queryUserInfoBySomeone" resultMap="userInfoMap" resultType="com.advancedc.hrsys.entity.UserInfo">
		SELECT
		ui.id,
		ui.name,
		ui.gender,
		ui.id_card,
		ui.is_married,
		ui.department_id,
		ui.position_id,
		ui.phone,
		ui.priority,
		ui.entry_time,
		ui.full_time,
		ui.created_time,
		ui.edited_time,
		jd.id jdid,
		jd.name jdname,
		jp.id jpid,
		jp.name jpname
		FROM
		hr_user_info ui
		INNER JOIN
		hr_job_department jd
		ON
		ui.department_id=jd.id
		INNER JOIN
		hr_job_position jp
		ON
		ui.position_id=jp.id		<where>
			<if test="someone.id>0">
			and ui.id = #{someone.id}			</if>
			<if test="someone.gender!=null">
			and ui.gender = #{someone.gender}			</if>
			<if test="someone.name!=null">
			and ui.name = #{someone.name}			</if>
			<if test="someone.idCard!=null">
			and ui.id_card = #{someone.idCard}			</if>
			<if test="someone.isMarried!=null">
			and ui.is_married = #{someone.isMarried}			</if>
			<if test="someone.jobDepartment!=null and someone.jobDepartment.id!=null">
			and ui.department_id = #{someone.jobDepartment.id}			</if>
			<if test="someone.jobPosition!=null and someone.jobPosition.id!=null">
			and ui.position_id = #{someone.jonPosition.id}			</if>
			<if test="someone.phone!=null">
			and ui.phone = #{someone.phone}			</if>
			<if test="someone.entryTime!=null">
			and ui.entry_time = #{someone.entryTime}			</if>
			<if test="someone.fullTime!=null">
			and ui.full_time = #{someone.fullTime}			</if>
		</where>
	</select>

上图用了INNER JOIN来查询看上去挺简洁的，有一种不简洁的写法如下，虽然也能得到结果，但是不知道性能对比如何

SELECT
		ui.id,
		ui.name,
		ui.gender,
		ui.id_card,
		ui.is_married,
		ui.department_id,
		ui.position_id,
		ui.phone,
		ui.priority,
		ui.entry_time,
		ui.full_time,
		ui.created_time,
		ui.edited_time,
		(select id jdid from hr_job_department jd where jd.id=ui.department_id) jdid,
		(select name jdname from hr_job_department jd where jd.id=ui.department_id) jdname,
		(select id jpid from hr_job_position jp where jp.id=ui.position_id) jpid,
		(select name jpname from hr_job_position jp where jp.id=ui.position_id) jpname
		FROM
		hr_user_info ui;