我们为什么要修改呢，这是因为我们查询的时候，我们有时候需要连表查询，例如我们需要查询出员工表的信息(emp_id,emp_name...)与此同时，我们还想查询出该员工所在的部门（dept_name）。此时，生成mapper文件查询的只是表中已有的字段。
所以我们可以在EmployeeMapper.xml中添加两个方法，
同时我们还要为Employee.java添加一个Department属性，别忘记增加setter和getter。
最后我们需要修改EmployeeMapper.xml文件，增加两个连表查询语句：
<?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.atguigu.crud.dao.EmployeeMapper">
<resultMap id="BaseResultMap" type="com.atguigu.crud.bean.Employee">
<!--
WARNING - @mbg.generated
This element is automatically generated by MyBatis Generator, do not modify.
This element was generated on Thu Oct 18 13:46:16 CST 2018.
-->
<id column="emp_id" jdbcType="INTEGER" property="empId" />
<result column="emp_name" jdbcType="VARCHAR" property="empName" />
<result column="gender" jdbcType="CHAR" property="gender" />
<result column="email" jdbcType="VARCHAR" property="email" />
<result column="d_id" jdbcType="INTEGER" property="dId" />
</resultMap>
<resultMap type="com.atguigu.crud.bean.Employee" id="WithDeptResultMap">
<id column="emp_id" jdbcType="INTEGER" property="empId" />
<result column="emp_name" jdbcType="VARCHAR" property="empName" />
<result column="gender" jdbcType="CHAR" property="gender" />
<result column="email" jdbcType="VARCHAR" property="email" />
<result column="d_id" jdbcType="INTEGER" property="dId" />
<association property="department" javaType="com.atguigu.crud.bean.Department">
<id column="dept_id" property="deptId"/>
<result column="dept_name" property="deptName"/>
</association>
</resultMap>
<sql id="Example_Where_Clause">
<!--
WARNING - @mbg.generated
This element is automatically generated by MyBatis Generator, do not modify.
This element was generated on Thu Oct 18 13:46:16 CST 2018.
-->
<where>
<foreach collection="oredCriteria" item="criteria" separator="or">
<if test="criteria.valid">
<trim prefix="(" prefixOverrides="and" suffix=")">
<foreach collection="criteria.criteria" item="criterion">
<choose>
<when test="criterion.noValue">
and ${criterion.condition}
</when>
<when test="criterion.singleValue">
and ${criterion.condition} #{criterion.value}
</when>
<when test="criterion.betweenValue">
and ${criterion.condition} #{criterion.value} and #{criterion.secondValue}
</when>
<when test="criterion.listValue">
and ${criterion.condition}
<foreach close=")" collection="criterion.value" item="listItem" open="(" separator=",">
#{listItem}
</foreach>
</when>
</choose>
</foreach>
</trim>
</if>
</foreach>
</where>
</sql>
<sql id="Update_By_Example_Where_Clause">
<!--
WARNING - @mbg.generated
This element is automatically generated by MyBatis Generator, do not modify.
This element was generated on Thu Oct 18 13:46:16 CST 2018.
-->
<where>
<foreach collection="example.oredCriteria" item="criteria" separator="or">
<if test="criteria.valid">
<trim prefix="(" prefixOverrides="and" suffix=")">
<foreach collection="criteria.criteria" item="criterion">
<choose>
<when test="criterion.noValue">
and ${criterion.condition}
</when>
<when test="criterion.singleValue">
and ${criterion.condition} #{criterion.value}
</when>
<when test="criterion.betweenValue">
and ${criterion.condition} #{criterion.value} and #{criterion.secondValue}
</when>
<when test="criterion.listValue">
and ${criterion.condition}
<foreach close=")" collection="criterion.value" item="listItem" open="(" separator=",">
#{listItem}
</foreach>
</when>
</choose>
</foreach>
</trim>
</if>
</foreach>
</where>
</sql>
<sql id="Base_Column_List">
<!--
WARNING - @mbg.generated
This element is automatically generated by MyBatis Generator, do not modify.
This element was generated on Thu Oct 18 13:46:16 CST 2018.
-->
emp_id, emp_name, gender, email, d_id
</sql>
<sql id="WithDept_Column_List">
e.emp_id, e.emp_name, e.gender, e.email, e.d_id,d.dept_id,d.dept_name
</sql>
<!-- 带部门的两个查询 -->
<select id="selectByExampleWithDept" resultMap="WithDeptResultMap">
select
<if test="distinct">
distinct
</if>
<include refid="WithDept_Column_List" />
from tbl_emp e left join tbl_dept d on e.d_id=d.dept_id
<if test="_parameter != null">
<include refid="Example_Where_Clause" />
</if>
<if test="orderByClause != null">
order by ${orderByClause}
</if>
</select>
<!-- 带部门的主键查询 -->
<select id="selectByPrimaryKeyWithDept" resultMap="WithDeptResultMap">
select
<include refid="WithDept_Column_List" />
from tbl_emp e left join tbl_dept d on e.d_id=d.dept_id where emp_id = #{empId,jdbcType=INTEGER} </select>
<select id="selectByExample" parameterType="com.atguigu.crud.bean.EmployeeExample" resultMap="BaseResultMap">
<!--
WARNING - @mbg.generated
This element is automatically generated by MyBatis Generator, do not modify.
This element was generated on Thu Oct 18 13:46:16 CST 2018.
-->
select
<if test="distinct">
distinct
</if>
<include refid="Base_Column_List" />
from tbl_emp
<if test="_parameter != null">
<include refid="Example_Where_Clause" />
</if>
<if test="orderByClause != null">
order by ${orderByClause}
</if>
</select>
<select id="selectByPrimaryKey" parameterType="java.lang.Integer" resultMap="BaseResultMap">
<!--
WARNING - @mbg.generated
This element is automatically generated by MyBatis Generator, do not modify.
This element was generated on Thu Oct 18 13:46:16 CST 2018.
-->
select
<include refid="Base_Column_List" />
from tbl_emp
where emp_id = #{empId,jdbcType=INTEGER}
</select>
<delete id="deleteByPrimaryKey" parameterType="java.lang.Integer">
<!--
WARNING - @mbg.generated
This element is automatically generated by MyBatis Generator, do not modify.
This element was generated on Thu Oct 18 13:46:16 CST 2018.
-->
delete from tbl_emp
where emp_id = #{empId,jdbcType=INTEGER}
</delete>
<delete id="deleteByExample" parameterType="com.atguigu.crud.bean.EmployeeExample">
<!--
WARNING - @mbg.generated
This element is automatically generated by MyBatis Generator, do not modify.
This element was generated on Thu Oct 18 13:46:16 CST 2018.
-->
delete from tbl_emp
<if test="_parameter != null">
<include refid="Example_Where_Clause" />
</if>
</delete>
<insert id="insert" parameterType="com.atguigu.crud.bean.Employee">
<!--
WARNING - @mbg.generated
This element is automatically generated by MyBatis Generator, do not modify.
This element was generated on Thu Oct 18 13:46:16 CST 2018.
-->
insert into tbl_emp (emp_id, emp_name, gender,
email, d_id)
values (#{empId,jdbcType=INTEGER}, #{empName,jdbcType=VARCHAR}, #{gender,jdbcType=CHAR},
#{email,jdbcType=VARCHAR}, #{dId,jdbcType=INTEGER})
</insert>
<insert id="insertSelective" parameterType="com.atguigu.crud.bean.Employee">
<!--
WARNING - @mbg.generated
This element is automatically generated by MyBatis Generator, do not modify.
This element was generated on Thu Oct 18 13:46:16 CST 2018.
-->
insert into tbl_emp
<trim prefix="(" suffix=")" suffixOverrides=",">
<if test="empId != null">
emp_id,
</if>
<if test="empName != null">
emp_name,
</if>
<if test="gender != null">
gender,
</if>
<if test="email != null">
email,
</if>
<if test="dId != null">
d_id,
</if>
</trim>
<trim prefix="values (" suffix=")" suffixOverrides=",">
<if test="empId != null">
#{empId,jdbcType=INTEGER},
</if>
<if test="empName != null">
#{empName,jdbcType=VARCHAR},
</if>
<if test="gender != null">
#{gender,jdbcType=CHAR},
</if>
<if test="email != null">
#{email,jdbcType=VARCHAR},
</if>
<if test="dId != null">
#{dId,jdbcType=INTEGER},
</if>
</trim>
</insert>
<select id="countByExample" parameterType="com.atguigu.crud.bean.EmployeeExample" resultType="java.lang.Long">
<!--
WARNING - @mbg.generated
This element is automatically generated by MyBatis Generator, do not modify.
This element was generated on Thu Oct 18 13:46:16 CST 2018.
-->
select count(*) from tbl_emp
<if test="_parameter != null">
<include refid="Example_Where_Clause" />
</if>
</select>
<update id="updateByExampleSelective" parameterType="map">
<!--
WARNING - @mbg.generated
This element is automatically generated by MyBatis Generator, do not modify.
This element was generated on Thu Oct 18 13:46:16 CST 2018.
-->
update tbl_emp
<set>
<if test="record.empId != null">
emp_id = #{record.empId,jdbcType=INTEGER},
</if>
<if test="record.empName != null">
emp_name = #{record.empName,jdbcType=VARCHAR},
</if>
<if test="record.gender != null">
gender = #{record.gender,jdbcType=CHAR},
</if>
<if test="record.email != null">
email = #{record.email,jdbcType=VARCHAR},
</if>
<if test="record.dId != null">
d_id = #{record.dId,jdbcType=INTEGER},
</if>
</set>
<if test="_parameter != null">
<include refid="Update_By_Example_Where_Clause" />
</if>
</update>
<update id="updateByExample" parameterType="map">
<!--
WARNING - @mbg.generated
This element is automatically generated by MyBatis Generator, do not modify.
This element was generated on Thu Oct 18 13:46:16 CST 2018.
-->
update tbl_emp
set emp_id = #{record.empId,jdbcType=INTEGER},
emp_name = #{record.empName,jdbcType=VARCHAR},
gender = #{record.gender,jdbcType=CHAR},
email = #{record.email,jdbcType=VARCHAR},
d_id = #{record.dId,jdbcType=INTEGER}
<if test="_parameter != null">
<include refid="Update_By_Example_Where_Clause" />
</if>
</update>
<update id="updateByPrimaryKeySelective" parameterType="com.atguigu.crud.bean.Employee">
<!--
WARNING - @mbg.generated
This element is automatically generated by MyBatis Generator, do not modify.
This element was generated on Thu Oct 18 13:46:16 CST 2018.
-->
update tbl_emp
<set>
<if test="empName != null">
emp_name = #{empName,jdbcType=VARCHAR},
</if>
<if test="gender != null">
gender = #{gender,jdbcType=CHAR},
</if>
<if test="email != null">
email = #{email,jdbcType=VARCHAR},
</if>
<if test="dId != null">
d_id = #{dId,jdbcType=INTEGER},
</if>
</set>
where emp_id = #{empId,jdbcType=INTEGER}
</update>
<update id="updateByPrimaryKey" parameterType="com.atguigu.crud.bean.Employee">
<!--
WARNING - @mbg.generated
This element is automatically generated by MyBatis Generator, do not modify.
This element was generated on Thu Oct 18 13:46:16 CST 2018.
-->
update tbl_emp
set emp_name = #{empName,jdbcType=VARCHAR},
gender = #{gender,jdbcType=CHAR},
email = #{email,jdbcType=VARCHAR},
d_id = #{dId,jdbcType=INTEGER}
where emp_id = #{empId,jdbcType=INTEGER}
</update>
</mapper>
现在，我们就能查询出带部门信息的员工表了。