mybatis之联表查询

今天碰到了一个问题，就是要在三张表里面各取一部分数据然后组成一个list传到前台页面显示。但是并不想在后台做太多判断，（因为涉及到for循环）会拉慢运行速度。正好用的框架是spring+springMVC+mybatis，所以很自然的就想到了联表查询。

一开始认为mybatis编写语句很简单，但是在编写的时候遇到了一些细节问题，所以发文记录一下。

先说一下背景：

框架：spring+springMVC+mybatis

表结构：

1、主表

2、从表

从表的uid对应主表的id，并将主表的id设为主键

接下来开始解析代码

先用mybatis的generator自动生成dao、mapper、model层

model层如下：

User.java

package am.model;

import java.util.List;

public class User {

    private Integer id;

    private String username;

    private String pwd;

    private List<Student> students;

    public Integer getId() {

        return id;

    }

    public void setId(Integer id) {

        this.id = id;

    }

    public String getUsername() {

        return username;

    }

    public void setUsername(String username) {

        this.username = username == null ? null : username.trim();

    }

    public String getPwd() {

        return pwd;

    }

    public void setPwd(String pwd) {

        this.pwd = pwd == null ? null : pwd.trim();

    }

        public List<Student> getStudents() {

                return students;

        }

        public void setStudents(List<Student> students) {

                this.students = students;

        }

}

　　

Student.java

package am.model;

public class Student {

    private Integer id;

    private String position;

    private String level;

    private Integer uid;

    public Integer getId() {

        return id;

    }

    public void setId(Integer id) {

        this.id = id;

    }

    public String getPosition() {

        return position;

    }

    public void setPosition(String position) {

        this.position = position == null ? null : position.trim();

    }

    public String getLevel() {

        return level;

    }

    public void setLevel(String level) {

        this.level = level == null ? null : level.trim();

    }

    public Integer getUid() {

        return uid;

    }

    public void setUid(Integer uid) {

        this.uid = uid;

    }

}

　　

需要注意的是在User表里面添加了成员变量

private List<Student> students;

　　

这是因为联表查询时Student表为从表，你最终要将得到的数据放在主表User里

mapping层

UserMapper.xml

<?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="am.dao.UserMapper" >

  <resultMap id="BaseResultMap" type="am.model.User" >

    <id column="id" property="id" jdbcType="INTEGER" />

    <result column="username" property="username" jdbcType="VARCHAR" />

    <result column="pwd" property="pwd" jdbcType="VARCHAR" />

  </resultMap>

  <resultMap id="studentForListMap" type="am.model.User" >

    <id column="id" property="id" jdbcType="INTEGER" />

    <result column="username" property="username" jdbcType="VARCHAR" />

    <result column="pwd" property="pwd" jdbcType="VARCHAR" />

    <collection property="students" javaType="java.util.List" ofType="am.model.Student"> 

            <result column="id" property="id" jdbcType="INTEGER" />

                    <result column="position" property="position" jdbcType="VARCHAR" />

                    <result column="level" property="level" jdbcType="VARCHAR" /> 

    </collection>

  </resultMap>

  <select id="studentForList" resultMap="studentForListMap" >

    select u.id,u.username,u.pwd,s.position,s.level from user u left join student s on u.id = s.uid

  </select>

  <sql id="Base_Column_List" >

    id, username, pwd

  </sql>

  <select id="selectByPrimaryKey" resultMap="BaseResultMap" parameterType="java.lang.Integer" >

    select

    <include refid="Base_Column_List" />

    from user

    where id = #{id,jdbcType=INTEGER}

  </select>

  <delete id="deleteByPrimaryKey" parameterType="java.lang.Integer" >

    delete from user

    where id = #{id,jdbcType=INTEGER}

  </delete>

  <insert id="insert" parameterType="am.model.User" >

    insert into user (id, username, pwd

      )

    values (#{id,jdbcType=INTEGER}, #{username,jdbcType=VARCHAR}, #{pwd,jdbcType=VARCHAR}

      )

  </insert>

  <insert id="insertSelective" parameterType="am.model.User" >

    insert into user

    <trim prefix="(" suffix=")" suffixOverrides="," >

      <if test="id != null" >

        id,

      </if>

      <if test="username != null" >

        username,

      </if>

      <if test="pwd != null" >

        pwd,

      </if>

    </trim>

    <trim prefix="values (" suffix=")" suffixOverrides="," >

      <if test="id != null" >

        #{id,jdbcType=INTEGER},

      </if>

      <if test="username != null" >

        #{username,jdbcType=VARCHAR},

      </if>

      <if test="pwd != null" >

        #{pwd,jdbcType=VARCHAR},

      </if>

    </trim>

  </insert>

  <update id="updateByPrimaryKeySelective" parameterType="am.model.User" >

    update user

    <set >

      <if test="username != null" >

        username = #{username,jdbcType=VARCHAR},

      </if>

      <if test="pwd != null" >

        pwd = #{pwd,jdbcType=VARCHAR},

      </if>

    </set>

    where id = #{id,jdbcType=INTEGER}

  </update>

  <update id="updateByPrimaryKey" parameterType="am.model.User" >

    update user

    set username = #{username,jdbcType=VARCHAR},

      pwd = #{pwd,jdbcType=VARCHAR}

    where id = #{id,jdbcType=INTEGER}

  </update>

</mapper>

　　

需要注意

1、property="students"中的students必须与User.java中的private List<Student> students;中的students命名一样

2、resultMap="studentForListMap"必须与<resultMap id="studentForListMap" type="am.model.User" >中的id一致

<resultMap id="studentForListMap" type="am.model.User" >

    <id column="id" property="id" jdbcType="INTEGER" />

    <result column="username" property="username" jdbcType="VARCHAR" />

    <result column="pwd" property="pwd" jdbcType="VARCHAR" />

    <collection property="students" javaType="java.util.List" ofType="am.model.Student"> 

            <result column="id" property="id" jdbcType="INTEGER" />

                    <result column="position" property="position" jdbcType="VARCHAR" />

                    <result column="level" property="level" jdbcType="VARCHAR" /> 

    </collection>

  </resultMap>

  <select id="studentForList" resultMap="studentForListMap" >

    select u.id,u.username,u.pwd,s.position,s.level from user u left join student s on u.id = s.uid

  </select>

　　

dao层

UserMapper.java

package am.dao;

import java.util.List;

import am.model.User;

public interface UserMapper {

    int deleteByPrimaryKey(Integer id);

    int insert(User record);

    int insertSelective(User record);

    User selectByPrimaryKey(Integer id);

    int updateByPrimaryKeySelective(User record);

    int updateByPrimaryKey(User record);

    List<User> queryForList();

    List<User> studentForList();

}

　　

spring-mybatis.xml

<?xml version="1.0" encoding="UTF-8"?>

<beans xmlns="http://www.springframework.org/schema/beans"

        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p"

        xmlns:context="http://www.springframework.org/schema/context"

        xmlns:mvc="http://www.springframework.org/schema/mvc"

        xsi:schemaLocation="http://www.springframework.org/schema/beans 

                        http://www.springframework.org/schema/beans/spring-beans-3.1.xsd 

                        http://www.springframework.org/schema/context 

                        http://www.springframework.org/schema/context/spring-context-3.1.xsd 

                        http://www.springframework.org/schema/mvc 

                        http://www.springframework.org/schema/mvc/spring-mvc-4.0.xsd">

        <!-- 自动扫描 -->

        <context:component-scan base-package="am.*" />

        <!-- 引入配置文件 -->

        <bean id="propertyConfigurer"

                class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">

                <property name="location" value="classpath:jdbc.properties" />

        </bean>

        <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource"

                destroy-method="close">

                <property name="driverClassName" value="${driver}" />

                <property name="url" value="${url}" />

                <property name="username" value="${username}" />

                <property name="password" value="${password}" />

                <!-- 初始化连接大小 -->

                <property name="initialSize" value="${initialSize}"></property>

                <!-- 连接池最大数量 -->

                <property name="maxActive" value="${maxActive}"></property>

                <!-- 连接池最大空闲 -->

                <property name="maxIdle" value="${maxIdle}"></property>

                <!-- 连接池最小空闲 -->

                <property name="minIdle" value="${minIdle}"></property>

                <!-- 获取连接最大等待时间 -->

                <property name="maxWait" value="${maxWait}"></property>

        </bean>

        <!-- spring和MyBatis完美整合，不需要mybatis的配置映射文件 -->

        <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">

                <property name="dataSource" ref="dataSource" />

                <!-- 自动扫描mapping.xml文件 -->

                <property name="mapperLocations" value="classpath:am/mapping/*.xml"></property>

        </bean>

        <!-- DAO接口所在包名，Spring会自动查找其下的类 -->

        <bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">

                <property name="basePackage" value="am.dao" />

                <property name="sqlSessionFactoryBeanName" value="sqlSessionFactory"></property>

        </bean>

        <!-- (事务管理)transaction manager, use JtaTransactionManager for global tx -->

        <bean id="transactionManager"

                class="org.springframework.jdbc.datasource.DataSourceTransactionManager">

                <property name="dataSource" ref="dataSource" />

        </bean>

</beans>

　　

我用serviceImp层来继承service层实现dao层，当然你也可以直接实现dao层，在这里就不多做赘述了。

分享一下项目包结构：

最后做一点简单的测试

MybatisController

package am.controller;

import java.util.List;

import javax.annotation.Resource;

import javax.servlet.http.HttpServletRequest;

import org.springframework.stereotype.Controller;

import org.springframework.web.bind.annotation.RequestMapping;

import org.springframework.web.servlet.ModelAndView;

import com.alibaba.fastjson.JSON;

import am.model.User;

import am.service.UserService;

@Controller 

@RequestMapping("/my")

public class MybatisController {

        @Resource

        private UserService userService;

        @RequestMapping("/t") 

         public ModelAndView test(HttpServletRequest request){ 

                 ModelAndView view = new ModelAndView("test");

                 List<User> users = userService.studentForList();

                 String s = JSON.toJSONString(users);

                 System.out.println(JSON.toJSONString(users));

                 return view;

         }

}

　　

success，得到想要数据。