多对一关系的处理
前期准备
- 数据库关系
可以看到employee是参照department表的，是多对一的关系
-
实体类
com.maple.pojo.Department
@Data public class Department { private Integer deptid; private String department_name; }com.maple.pojo.Employee
@Data public class Employee { private Integer empid; private String last_name; private String email; private Integer gender; private Integer deptid; private Date birth; }com.maple.entity.Department
@Data public class Department extends com.maple.pojo.Department { List<Employee> employees; @Override public String toString(){ return super.toString()+"employees=" + employees; } }com.maple.entity.Employee
@Data public class Employee extends com.maple.pojo.Employee { private Department department; @Override public String toString(){ return super.toString()+"department=" + department; } } -
核心配置文件
<?xml version="1.0" encoding="UTF-8" ?> <!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd"> <configuration> <properties resource="DB.properties"></properties> <settings> <setting name="logImpl" value="LOG4J"/> </settings> <environments default="development"> <environment id="development"> <transactionManager type="JDBC"/> <dataSource type="POOLED"> <property name="driver" value="${driver}"/> <property name="url" value="${url}"/> <property name="username" value="${username}"/> <property name="password" value="${password}"/> </dataSource> </environment> </environments> <mappers> <mapper resource="com/maple/mapper/DepMapper.xml"/> <mapper resource="com/maple/mapper/EmpMapper.xml"/> </mappers> </configuration> -
Mapper接口
public interface EmpMapper { List<Employee> queryEmpList(); Employee queryEmpById(Integer empid); } -
测试类
@Test public void test2(){ SqlSession sqlSession = MyBatisUtil.getSqlSession(); EmpMapper empMapper = sqlSession.getMapper(EmpMapper.class); List<Employee> empList = empMapper.queryEmpList(); empList.forEach(emp-> System.out.println(emp)); } @Test public void test3(){ SqlSession sqlSession = MyBatisUtil.getSqlSession(); EmpMapper empMapper = sqlSession.getMapper(EmpMapper.class); Employee employee = empMapper.queryEmpById(1001); System.out.println(employee); }
使用resultType返回类型查询
mapper文件的select
<select id="queryEmpList" resultType="com.maple.entity.Employee">
select * from mybatis.employee,mybatis.department
where employee.deptid=department.deptid
# select * from mybatis.employee
</select>
执行结果
可以看到department并没有映射到结果
解决方法
-
按照结果嵌套处理(使用resultMap返回类型，通过子查询获取department)
-
EmpMapper.xml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"> <mapper namespace="com.maple.mapper.EmpMapper"> <!-- 在核心配置文件中用<typeAliases>导入Employee所在包 --> <resultMap id="emp" type="com.maple.entity.Employee"> <!-- 省略了能自动映射的列 --> <!-- 复杂的属性需要单独处理 对象:association 集合:collection --> <association property="department" column="deptid" javaType="com.maple.pojo.Department" select="queryDept"/> </resultMap> <select id="queryEmpList" resultMap="emp"> select * from mybatis.employee </select> <select id="queryDept" resultType="com.maple.pojo.Department"> select * from mybatis.department where deptid = #{deptid} </select> </mapper> -
执行结果
-
-
按照结果嵌套处理(使用连表查询)
EmpMapper.xml核心代码
<select id="queryEmpById" resultMap="emp2" parameterType="int"> select empid, last_name, email, gender, employee.deptid, birth , department_name from mybatis.department ,mybatis.employee where employee.deptid=department.deptid and empid=#{empid} </select> <resultMap id="emp2" type="com.maple.entity.Employee"> <result property="empid" column="empid"/> <result property="last_name" column="last_name"/> <result property="email" column="email"/> <result property="gender" column="gender"/> <result property="deptid" column="deptid"/> <result property="birth" column="birth"/> <association property="department" javaType="com.maple.pojo.Department"> <result property="deptid" column="deptid"/> <result property="department_name" column="department_name"/> </association> </resultMap>运行结果
一对多关系的处理
接口
public interface DepMappper {
List<Department> queryDepList();
List<Department> queryDepById(@Param("deptid") int deptid);
}
DepMappper.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.maple.mapper.DepMappper">
<resultMap id="dept" type="com.maple.entity.Department">
<result property="deptid" column="deptid"/>
<result property="department_name" column="department_name"/>
<collection property="employees" ofType="com.maple.pojo.Employee">
<result property="empid" column="empid"/>
<result property="last_name" column="last_name"/>
<result property="email" column="email"/>
<result property="gender" column="gender"/>
<result property="deptid" column="deptid"/>
<result property="birth" column="birth"/>
</collection>
</resultMap>
<select id="queryDepList" resultMap="dept">
select d.department_name,e.* from mybatis.department d,mybatis.employee e where e.deptid=d.deptid
</select>
<select id="queryDepById" resultMap="dept">
select d.department_name,e.* from mybatis.department d,mybatis.employee e where e.deptid=d.deptid and d.deptid=#{deptid}
</select>
</mapper>
测试类
@Test
public void test3(){
SqlSession sqlSession = MyBatisUtil.getSqlSession();
DepMappper depMappper = sqlSession.getMapper(DepMappper.class);
List<Department> deptList = depMappper.queryDepList();
deptList.forEach(dept-> System.out.println(dept));
}
@Test
public void test4(){
SqlSession sqlSession = MyBatisUtil.getSqlSession();
DepMappper depMappper = sqlSession.getMapper(DepMappper.class);
List<Department> deptList = depMappper.queryDepById(102);
deptList.forEach(dept-> System.out.println(dept));
}
运行结果
