mybatis源码阅读-高级用法(二)

新建学生表和学生证表

--学生表
CREATE TABLE student(
 id INT NOT NULL PRIMARY KEY AUTO_INCREMENT COMMENT 'id',
 `name` VARCHAR(20) NOT NULL  COMMENT '姓名',
 `age` INT NOT NULL COMMENT '年龄',
  sex INT NOT NULL COMMENT '性别 1 男 0 女',
  cid INT NOT NULL COMMENT '班级id',
  cardId INT COMMENT '学生证id'
 )
 --学生证表
 CREATE TABLE Card
 (
  id INT NOT NULL PRIMARY KEY AUTO_INCREMENT COMMENT 'id',
  number INT NOT NULL COMMENT '学生证id',
  studentId INT NOT NULL COMMENT '学生id'
 )

动态标签

include&sql

 实现sql复用

<sql id="columns">
        id,name,age
    </sql>
    <!--useCache 表示使用缓存 -->
    <select id="selectOne" parameterType="string"  flushCache="false"
        useCache="true" resultMap="studentAndClassMap">
        select <include refid="columns"/> from student
        <!--模拟if标签 -->
        <if test="id !=null and id!=''">
            where id=#{id}
        </if>
    </select>

where&if

<select id="selectOne" parameterType="string"  flushCache="false"
        useCache="true" resultMap="studentAndClassMap">
        select <include refid="columns"/> from student
        <where>
            <if test="id !=null and id!=''">
               and id=#{id}
            </if>
            <if test="name !=null and name!=''">
               and name=#{name}
            </if>
        </where>
    
        
    </select>

where标签里面有if标签成立 则自动在sql后面拼接where 同时去除多余的and

choose, when, otherwise

<select id="find" parameterType="map" resultType="student">

        select * from student  
    <!--     if else if  else -->
    <where>
        <choose>
            <when test="sex != null">
              and sex=#{sex}
           </when>
           <when test="name != null and name != ''">
               and name like concat('%',#{name},'%')
           </when>
        <!-- <otherwise>
           ...
        </otherwise> -->
        </choose>
    </where>
    </select>

trim, set

select * from user 

　　<trim prefix="WHERE" prefixoverride="AND |OR">

　　　　<if test="name != null and name.length()>0"> AND name=#{name}</if>

　　　　<if test="gender != null and gender.length()>0"> AND gender=#{gender}</if>

　　</trim>

满足条件拼接 where 同时去掉前面多余的and或者or

　update user

　　<trim prefix="set" suffixoverride="," suffix=" where id = #{id} ">

　　　　<if test="name != null and name.length()>0"> name=#{name} , </if>

　　　　<if test="gender != null and gender.length()>0"> gender=#{gender} ,  </if>

　　</trim>

　　满足条件拼接set 同时去掉后面多余的， 再拼接 suffix

　update user

　　<set>

　　　　<if test="name != null and name.length()>0"> name=#{name} , </if>

　　　　<if test="gender != null and gender.length()>0"> gender=#{gender} ,  </if>

　　</set>

满足条件拼接set 同时去掉多余的,

一对多级联

1.学生mapper增加一个根据班级获取学生

<select id="findByClassesId" parameterType="int" resultType="student">
        select * from student where cid=#{id}
    </select>

 public List<Student> findByStudent(Student params);

2.在classes类增加一个学生集合

public class Classes implements Serializable{
....
private List<Student> students;
}

2.classesMapper增加自定义resultMap

<resultMap id="classVoMap" type="com.liqiang.vo.ClassesVo">
        <id property="id" column="id" />
        <result property="name" column="name" />
        <collection property="students" column="id"
            select="com.liqiang.mapper.StudentMapper.findByClassesId"></collection>
    </resultMap>

select为调用指定mapperStatement(可以理解是指定key的标签namespace+id)
column 为将哪个列作为参数传递
students 返回结果赋值的属性
可以设置 fetchType="eager(默认值 不延迟加载)|lazy"  是否延迟加载的意思

3.在需要使用resultMap的地方指定resultMap 如根据id获得class

<select id="findById" parameterType="Integer" resultMap="classVoMap">
   select *
   from classes where id=#{id}
</select>

多对一， 一对一级联

1.学生表增加classes

public class Student implements Serializable{
    ....
    private Classes classes;
}

2.学生表增加自定义resultMap

    <resultMap id="studentAndClassMap" type="student" >
        <id property="id" column="id" />
        <result property="name" column="name" />
        <result property="age" column="age" />
        <result property="cid" column="cid" />
        <association property="classes" column="cid"
                     select="com.liqiang.mapper.ClassesMapper.findById"></association>
    </resultMap>

3.在需要级联的地方resultMap改为这个

    <!--useCache 表示使用缓存 -->
    <select id="selectOne" parameterType="string"  flushCache="false"
        useCache="true" resultMap="studentAndClassMap">
        select * from student
        <!--模拟if标签 -->
        <if test="id !=null and id!=''">
            where id=#{id}
        </if>
    </select>

跟多对一一样 只是标签改为association

resultMap或者ParameterMap继承

<resultMap id="studentMap" type="studentVo">
        <id property="id" column="id" />
        <result property="name" column="name" />
        <result property="age" column="age" />
        <result property="cid" column="cid" />
        
        <association property="card" column="id"
            select="com.liqiang.mapper.CardMapper.findByStudentId"></association>
    </resultMap>
    <resultMap id="studentAndClassMap" type="student" >
        <id property="id" column="id" />
        <result property="name" column="name" />
        <result property="age" column="age" />
        <result property="cid" column="cid" />
        <association property="classes" column="cid"
                     select="com.liqiang.mapper.ClassesMapper.findById"></association>
    </resultMap>

如我定义了2个Resultmap 一个需要class级联 一个只需要学生证级联，是否发现前面几个属性几乎是copy的

我们可以这样

<resultMap id="simpleType" type="student">
        <id property="id" column="id" />
        <result property="name" column="name" />
        <result property="age" column="age" />
        <result property="cid" column="cid" />

    </resultMap>
    <resultMap id="studentMap" type="student" extends="simpleType">

        <association property="card" column="id"
            select="com.liqiang.mapper.CardMapper.findByStudentId"></association>
    </resultMap>
    <resultMap id="studentAndClassMap" type="student"  extends="simpleType" >

        <association property="classes" column="cid"
                     select="com.liqiang.mapper.ClassesMapper.findById"></association>
    </resultMap>

使用extends="simpleType" 继承