准备
数据库表
员工表（t_employee）
部门表（t_department）
表所对应的类
员工 t_employee
部门 t_department
方式一（级联方式处理映射关系）
以通过员工id查询员工信息以及部门信息为例。
接口
public interface EmployeeMapper { //通过员工的eid查询员工的所有信息 Employee selectEmployee(@Param("eid")Integer eid); }
映射文件
<?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"><!--约束，约束不同xml中所写的标签也不同--> <mapper namespace="com.xxx.mapper.EmployeeMapper"><!--接口--> <resultMap id="empMap" type="com.xxx.pojo.Employee"> <id property="eid" column="eid"></id> <result property="ename" column="ename"></result> <result property="age" column="age"></result> <result property="sex" column="sex"></result> <result property="dept.did" column="did"></result> <result property="dept.dname" column="dname"></result> </resultMap> <select id="selectEmployee" resultMap="empMap"> SELECT t_employee.*,t_department.* FROM t_employee LEFT JOIN t_department ON t_employee.did=t_department.did WHERE t_employee.eid=#{eid} </select> </mapper>
测试
EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class); //查询员工eid=1的所有信息 Employee employee = mapper.selectEmployee(1); System.out.println(employee);
方式二（使用association处理映射关系）
以通过员工id查询员工信息以及部门信息为例。
接口
public interface EmployeeMapper { //通过员工的eid查询员工的所有信息 Employee selectEmployee(@Param("eid")Integer eid); }
映射文件
<?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"><!--约束，约束不同xml中所写的标签也不同--> <mapper namespace="com.xxx.mapper.EmployeeMapper"><!--接口--> <resultMap id="empMap" type="com.xxx.pojo.Employee"> <id property="eid" column="eid"></id> <result property="ename" column="ename"></result> <result property="age" column="age"></result> <result property="sex" column="sex"></result> <association property="dept" javaType="com.xxx.pojo.Department"> <id property="did" column="did"></id> <result property="dname" column="dname"></result> </association> </resultMap> <select id="selectEmployee" resultMap="empMap"> SELECT t_employee.*,t_department.* FROM t_employee LEFT JOIN t_department ON t_employee.did=t_department.did WHERE t_employee.eid=#{eid} </select> </mapper>
测试
EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class); //查询员工eid=1的所有信息 Employee employee = mapper.selectEmployee(1); System.out.println(employee);
方式三（分步查询）
以通过员工id查询员工信息以及部门信息为例。
接口
第一步接口
public interface EmployeeMapper { //通过员工的did查询员工的所有信息 Employee selectEmployee(@Param("eid")Integer eid); }
第二步接口
public interface DepartmentMapper { //通过员工的did查询员工的所有信息 Department selectDepartment(@Param("did")Integer did); }
映射文件
第一步映射文件
<?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"><!--约束，约束不同xml中所写的标签也不同--> <mapper namespace="com.xxx.mapper.EmployeeMapper"><!--接口--> <resultMap id="empMap" type="com.xxx.pojo.Employee"> <id property="eid" column="eid"></id> <result property="ename" column="ename"></result> <result property="age" column="age"></result> <result property="sex" column="sex"></result> <!--此处的select中写的是第二步接口中的方法的全路径，colum中写的是第一步查出的那个数据作为第二步的参数--> <association property="dept" select="com.xxx.mapper.DepartmentMapper.selectDepartment" column="did"> </association> </resultMap> <select id="selectEmployee" resultMap="empMap"> SELECT t_employee.*,t_department.* FROM t_employee LEFT JOIN t_department ON t_employee.did=t_department.did WHERE t_employee.eid=#{eid} </select> </mapper>
第二步映射文件
<?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"><!--约束，约束不同xml中所写的标签也不同--> <mapper namespace="com.xxx.mapper.DepartmentMapper"><!--接口--> <select id="selectDepartment" resultType="com.xxx.pojo.Department"> select * from t_department where did=#{did} </select> </mapper>
测试
EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class); //查询员工eid=1的所有信息 Employee employee = mapper.selectEmployee(1); System.out.println(employee);
