MyBatis--resultMap

逃离我推掉我的手 2023-06-07 12:25 10阅读 0赞

# *resultMap:自定义结果集映射规则* #

# 1.级联属性封装结果集 #

##  1.1 方式一 ##

<mapper namespace="dao.StudentMapper">
        <resultMap  type="bean.Student" id="MyStudent">
            <!-column：指定哪一列,property：指定对应的javaBean属性-->
            <id column="student_id" property="id"/>
            
            <result column="student_name" property="studentName"/>
    
            
            <result column="sex" property="sex"/>
    
            
            <result column="school_id" property="school.id"/>
            <result column="school_name" property="school.schoolName"/>
        </resultMap>
    
        <select id="getStudent" resultMap="MyStudent">
            SELECT * FROM student,school WHERE student_id=#{
        id} AND student.school_name=school.school_name
        </select>
    </mapper>

##  1.2 方式二（association） ##

<mapper namespace="dao.StudentMapper">
        <resultMap  type="bean.Student" id="MyStudent2">
            <id column="student_id" property="id"/>
            <result column="student_name" property="studentName"/>
            <result column="sex" property="sex"/>
    
            
            <association property="school" javaType="bean.School">
                <id column="school_id" property="id"/>
                <result column="school_name" property="schoolName"/>
            </association>
    
        </resultMap>
    
        <select id="getStudent" resultMap="MyStudent2">
            SELECT * FROM student,school WHERE student_id=#{
        id} AND student.school_name=school.school_name
        </select>
    </mapper>

# 2.分步查询 #

根据学生的学号查询学校的信息  
  分步–>1.根据学生学号查询学校姓名  
    -->2.根据学校姓名查询学校信息

<mapper namespace="dao.StudentMapper">
    
        <resultMap id="MyStudent3" type="bean.Student">
            <id column="student_id" property="id"/>
            <result column="student_name" property="studentName"/>
            <result column="sex" property="sex"/>
            
    		
            <association property="school" select="dao.SchoolMapper.getSchool" column="school_name"></association>
        </resultMap>
    
        <select id="getStudent2" resultMap="MyStudent3">
            SELECT * FROM student WHERE student_id=#{
        id}
        </select>
    
    </mapper>

# 3.延迟加载 #

延迟加载（懒加载）：(按需加载)
     	Student==>School：
     		查询Student对象的时候，都将一起查询出来。
     		学校信息在我们使用的时候再去查询；
     		分段查询的基础之上在mybatis-config.xml加上两个配置：

# 4.collection标签定义的封装规则 #

<resultMap id="MySchool2" type="bean.School">
        <id column="school_id" property="id"/>
        <result column="school.school_name" property="schoolName"/>
    
        
        <collection property="students" ofType="bean.Student">
            
            <id column="student_id" property="id"/>
            <result column="student_name" property="studentName"/>
            <result column="sex" property="sex"/>
        </collection>
    </resultMap>

扩展：多列的值传递过去： 将多列的值封装map传递；
    			 column="{key1=column1,key2=column2}"

fetchType="lazy"：表示使用延迟加载； - lazy：延迟 - eager：立即

5.鉴别器（了解）  
  鉴别器：mybatis可以使用discriminator判断某列的值，然后根据某列的值改变封装行为

如果查询结果的student\_name是litao则使其sex映射到student\_name