mybatis sql 知识点

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/u011487710/article/details/79731666

1.association 持久化对象组合对象时，被组合的对象用association标签

<?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.queen.mybatis.mapper.UserMapper">

    <resultMap type="com.queen.mybatis.bean.User" id="userResultMap">
        <id property="id" column="id"/>
        <result property="loginId" column="loginId" />
        <result property="userName" column="userName"/>
        <result property="note" column="note"/>
            <!--assocication可以指定联合的JavaBean对象 
                property="role"指定哪个属性是联合的对象
                javaType:指定这个属性对象的类型
            -->
        <association property="role" javaType="com.queen.mybatis.bean.Role">
            <id column="role_id" property="id"/>
            <result column="roleName" property="roleName"/>
        </association>
    </resultMap>

    <select id="getUserById" resultMap="userResultMap">
        select m.id id, m.loginId loginId, m.userName userName, m.roleId roleId,m.note note, n.id role_id, n.roleName roleName 
            from t_user m left join t_role n on m.roleId=n.id
            where m.id=#{id}
    </select>

</mapper>

2.Collection 持久化对象与被组合对象是一对多关系时

<resultMap id="ClazzResultMap" type="com.cn.hnust.pojo.ClazzEntity" >
    <id column="classID" property="clazzID" jdbcType="INTEGER" />
    <result column="className" property="clazzName" jdbcType="VARCHAR" />
    <collection property="studentList" column="classID" javaType="ArrayList" 
                ofType="com.cn.hnust.pojo.StudentEntity" select="getStudentByClassID"/>
  </resultMap>

  <resultMap id="StudentResultMap" type="com.cn.hnust.pojo.StudentEntity">  
    <id property="stuID" column="stuID" />  
    <result property="stuName" column="stuName" />  
    <result property="stuAge" column="stuAge" />  
    <result property="stuAddress" column="stuAddress" />
  </resultMap>  

<select id="getClassByID" resultMap="ClazzResultMap" parameterType="java.lang.Integer" >
    select classID,className
    from class_t
    where classID = #{clazzID}
</select>

<select id="getStudentByClassID" resultMap="StudentResultMap" parameterType="java.lang.Integer" >
    select stuID,stuName,stuAge,stuAddress,classID
    from student_t
    where classID = #{clazzID}
</select>

3.extends 标签的使用
代表resultMap CocBeanResult 中包含resultMap CocBeanResult的所有字段

<resultMap id="CocBeanResult" type="CocBean">  
    <result property="name" column="NAME"/>  
    <result property="volume" column="VOLUME"/>  
</resultMap>  

<resultMap id="simpleRow" type="CocTreeNode" extends="CocBeanResult">  
    <result property="level1" column="LEVEL1"/>  
    <result property="level2" column="LEVEL2"/>  
</resultMap>