MyBatis中＜collection＞的两种使用方法，及效率比较

Dear 丶 2022-07-15 15:09 504阅读 0赞

**方案一**

<resultMap type="Student" id="StudentMap">
              <id column="id" property="id" />
              <result column="name" property="name" />
              <result column="job" property="job" />
              <collection property="scores" ofType="Score" column="id"  select="queryScoresBySID" ></collection>
         </resultMap>
         <resultMap type="Score" id="ScoreMap">
              <id column="id" property="id" />
              <result column="num" property="num" />
              <association property="subject" javaType="Subject" column="subject" select="querySubjectBySubId"></association>
         </resultMap>
         <select id="queryStudents" resultMap="StudentMap" >
              SELECT id,name,job FROM t_student
         </select>
         <select id="queryScoresBySID" resultMap="ScoreMap">
              SELECT id,num,subject FROM t_score WHERE sid = #{sid}
         </select>
         <select id="querySubjectBySubId" resultType="Subject" >
              SELECT id,name FROM t_subject where id = #{id}
         </select>

**方案二**

<resultMap type="Student" id="StudentMap2">
              <id column="id" property="id" />
              <result column="name" property="name" />
              <result column="job" property="job" />
              <collection property="scores" javaType="java.util.ArrayList" ofType="Score">
                  <id column="id" property="id" />
                  <result column="num" property="num" />
                  <association property="subject" javaType="Subject">
                       <id column="id" property="id" />
                       <result column="name" property="name" />
                  </association>
              </collection>
         </resultMap>
         <select id="queryStudents2" resultMap="StudentMap2" >
              SELECT stu.id,stu.name name,stu.job,sco.id id,sco.num num,sub.id id,sub.name name
              FROM t_student stu LEFT JOIN t_score sco ON stu.id = sco.sid LEFT JOIN t_subject sub ON sco.subject = sub.id
         </select>

\*\* 比较\*\*

方案一：需要执行至少三次sql语句，开启三次事务才能完成本次请求。
    方案二：需要执行一次sql语句，开启一次事务就能完成本次请求
         
    方案二比方案一的效率要高，但是在使用的时候，方案一的代码可重用性要高
    
    如果想要追求代码重用性可以选择方案一
    如果比较在乎运行的性能可以选择方案二