MyBatis联表查询——别名方式
在使用MyBatis你想工程时，单表操作其实是非常完美的，涉及到多表联合查询时，需要我们自己写联表的SQL语句。
我拿出项目中的部分代码作为示例，
EmployeeMapper.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="com.atguigu.crud.mapper.EmployeeMapper">
<resultMap id="BaseResultMap"
type="com.atguigu.crud.bean.Employee">
<result 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>
<sql id="Example_Where_Clause">
<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">
<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">
emp_id, emp_name, gender, email, d_id
</sql> <!-- 别名 -->
<sql id="Base_Column_List_Alias">
${alias}.emp_id, ${alias}.emp_name, ${alias}.gender, ${alias}.email, d_id
</sql> <!-- 关联对象 -->
<resultMap type="Employee" id="BaseResultMapWithDept" extends="BaseResultMap">
<association property="department" resultMap="com.atguigu.crud.mapper.DepartmentMapper.BaseResultMap"></association>
</resultMap> 说明：department为Employee.java实体类中的关联对象，private Department department;
<!-- 查询返回部门信息 -->
<select id="selectByExampleWithDept" resultMap="BaseResultMapWithDept">
select
<if test="distinct">
distinct
</if>
<include refid="Base_Column_List_Alias">
<property name="alias" value="e" />
</include>
,
<include refid="com.atguigu.crud.mapper.DepartmentMapper.Base_Column_List_Alias">
<property name="alias" value="d" />
</include> 说明：com.atguigu.crud.mapper.DepartmentMapper.Base_Column_List_Alias为DepartmentMapper.xml中的字段别名形式。
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> <select id="selectByExample"
parameterType="com.atguigu.crud.bean.EmployeeExample"
resultMap="BaseResultMap">
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>
<delete id="deleteByExample"
parameterType="com.atguigu.crud.bean.EmployeeExample">
delete from tbl_emp
<if test="_parameter != null">
<include refid="Example_Where_Clause" />
</if>
</delete>
<insert id="insert"
parameterType="com.atguigu.crud.bean.Employee">
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">
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">
select count(*) from tbl_emp
<if test="_parameter != null">
<include refid="Example_Where_Clause" />
</if>
</select>
<update id="updateByExampleSelective" parameterType="map">
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">
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>
</mapper>
DepartmentMapper.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="com.atguigu.crud.mapper.DepartmentMapper">
<resultMap id="BaseResultMap" type="com.atguigu.crud.bean.Department">
<result column="dept_id" jdbcType="INTEGER" property="deptId" />
<result column="dept_name" jdbcType="VARCHAR" property="deptName" />
</resultMap>
<sql id="Example_Where_Clause">
<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">
<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">
dept_id, dept_name
</sql> <!-- 别名 -->
<sql id="Base_Column_List_Alias">
${alias}.dept_id, ${alias}.dept_name
</sql> <select id="selectByExample" parameterType="com.atguigu.crud.bean.DepartmentExample" resultMap="BaseResultMap">
select
<if test="distinct">
distinct
</if>
<include refid="Base_Column_List" />
from tbl_dept
<if test="_parameter != null">
<include refid="Example_Where_Clause" />
</if>
<if test="orderByClause != null">
order by ${orderByClause}
</if>
</select>
<delete id="deleteByExample" parameterType="com.atguigu.crud.bean.DepartmentExample">
delete from tbl_dept
<if test="_parameter != null">
<include refid="Example_Where_Clause" />
</if>
</delete>
<insert id="insert" parameterType="com.atguigu.crud.bean.Department">
insert into tbl_dept (dept_id, dept_name)
values (#{deptId,jdbcType=INTEGER}, #{deptName,jdbcType=VARCHAR})
</insert>
<insert id="insertSelective" parameterType="com.atguigu.crud.bean.Department">
insert into tbl_dept
<trim prefix="(" suffix=")" suffixOverrides=",">
<if test="deptId != null">
dept_id,
</if>
<if test="deptName != null">
dept_name,
</if>
</trim>
<trim prefix="values (" suffix=")" suffixOverrides=",">
<if test="deptId != null">
#{deptId,jdbcType=INTEGER},
</if>
<if test="deptName != null">
#{deptName,jdbcType=VARCHAR},
</if>
</trim>
</insert>
<select id="countByExample" parameterType="com.atguigu.crud.bean.DepartmentExample" resultType="java.lang.Long">
select count(*) from tbl_dept
<if test="_parameter != null">
<include refid="Example_Where_Clause" />
</if>
</select>
<update id="updateByExampleSelective" parameterType="map">
update tbl_dept
<set>
<if test="record.deptId != null">
dept_id = #{record.deptId,jdbcType=INTEGER},
</if>
<if test="record.deptName != null">
dept_name = #{record.deptName,jdbcType=VARCHAR},
</if>
</set>
<if test="_parameter != null">
<include refid="Update_By_Example_Where_Clause" />
</if>
</update>
<update id="updateByExample" parameterType="map">
update tbl_dept
set dept_id = #{record.deptId,jdbcType=INTEGER},
dept_name = #{record.deptName,jdbcType=VARCHAR}
<if test="_parameter != null">
<include refid="Update_By_Example_Where_Clause" />
</if>
</update>
</mapper>
该别名的定义，是为了在EmployeeMapper.xml以别名方式进行查询。
实体类：
Department.java：
package com.atguigu.crud.bean;
public class Employee {
private Integer empId;
private String empName;
private String gender;
private String email;
private Integer dId;
// 希望查询员工的同时部门信息也是查询好的
private Department department;
public Employee() {
super();
// TODO Auto-generated constructor stub
}
public Employee(Integer empId, String empName, String gender, String email, Integer dId) {
super();
this.empId = empId;
this.empName = empName;
this.gender = gender;
this.email = email;
this.dId = dId;
}
public Integer getEmpId() {
return empId;
}
public void setEmpId(Integer empId) {
this.empId = empId;
}
public String getEmpName() {
return empName;
}
public void setEmpName(String empName) {
this.empName = empName == null ? null : empName.trim();
}
public String getGender() {
return gender;
}
public void setGender(String gender) {
this.gender = gender == null ? null : gender.trim();
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email == null ? null : email.trim();
}
public Integer getdId() {
return dId;
}
public void setdId(Integer dId) {
this.dId = dId;
}
public Department getDepartment() {
return department;
}
public void setDepartment(Department department) {
this.department = department;
}
}
Department.java：
package com.atguigu.crud.bean;
public class Department {
private Integer deptId;
private String deptName;
// 一旦生成有参的构造器，那么就一定要生成有参的构造器
public Department() {
super();
// TODO Auto-generated constructor stub
}
public Department(Integer deptId, String deptName) {
super();
this.deptId = deptId;
this.deptName = deptName;
}
public Integer getDeptId() {
return deptId;
}
public void setDeptId(Integer deptId) {
this.deptId = deptId;
}
public String getDeptName() {
return deptName;
}
public void setDeptName(String deptName) {
this.deptName = deptName == null ? null : deptName.trim();
}
}
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