mybatis多对一一对多的关联及拼接操作以及缓存处理

1、多对一：简单的说，就是将查询出来的多个结果分别赋予另一个结果的三个属性

多对一关联：

①创建对象，添加属性（这里不再展示config.xml文件连接数据库，这是每一个项目必备的）

@Data
public class Emp {
    private Integer empNo;
    private String ename;
    private String job;
    private Integer mgr;
    private String hireDate1;
    private Double sal;
    private Double comm;
    private Integer deptNo;
    public Integer state;
    //描述多对一的关系
    private Dept dept;
}

@Data
public class Dept {
    private Integer deptNo;
    private String dname;
    private String loc;
}

 ②建立接口，创建方法

public interface EmpMapper {
     //描述多对一的关联
    Emp findById1(int id);
}

③xml文件书写SQL查询语句 

    
    
        
        
        
        
            
            
            
        
    
    
        select * from emp e left join dept d on e.deptno=d.deptno where state=1 and empno=#{id}
    

④ 实现多对一关联

    private static void b1(){
        InputStream inputStream = Main.class.getClassLoader().getResourceAsStream("config.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        EmpMapper empMapper = sqlSession.getMapper(EmpMapper.class);
        Emp emp = empMapper.findById1(7980);
        System.out.println(emp);
        sqlSession.close();
    }

 多对一拼接：

①建立接口，创建方法

public interface EmpMapper {
    //描述多对一的拼接操作
    Emp findById2(int id);
}

public interface DeptMapper {
    //多对一的，查询出来的部门信息
    Dept findById(int id);
}

 ②xml文件书写SQL查询语句 

    
        select * from dept where DEPTNO = #{id}
    

 ③实现多对一拼接

    private static void b2(){
        InputStream inputStream = Main.class.getClassLoader().getResourceAsStream("config.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        EmpMapper empMapper = sqlSession.getMapper(EmpMapper.class);
        //只能获取到员工信息
        Emp emp = empMapper.findById2(7980);
        //只能查询到部门信息
        DeptMapper deptMapper = sqlSession.getMapper(DeptMapper.class);
        Dept dept = deptMapper.findById(emp.getDeptNo());
        //组装
        emp.setDept(dept);
        System.out.println(emp);
        sqlSession.close();
    }

2、一对多：相当于一个部门拥有多个员工

一对多关联：

①创建对象，添加属性

@Data
public class Dept {
    private Integer deptNo;
    private String dname;
    private String loc;
    //描述一对多关系
    private Set emp;
}

@Data
public class Emp {
    private Integer empNo;
    private String ename;
    private String job;
    private Integer mgr;
    private String hireDate1;
    private Double sal;
    private Double comm;
    private Integer deptNo;
    public Integer state;
}

②建立接口，创建方法

public interface DeptMapper {
    //一对多的关联写法
    Dept findById1(int id);
}

③xml文件书写SQL查询语句 

    
    
        
        
        
    
    
        select * from dept d left join emp e on d.deptno = e.deptno where d.deptno = #{id}
    

④ 实现一对多关联

    private static void c1(){
        InputStream inputStream = Main.class.getClassLoader().getResourceAsStream("config.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        DeptMapper deptMapper = sqlSession.getMapper(DeptMapper.class);
        Dept dept = deptMapper.findById1(10);
        System.out.println(dept);
        sqlSession.close();
    }

一对多拼接：

①建立接口，创建方法

public interface DeptMapper {
    //一对多的拼接写法
    Dept findById2(int id);
}

public interface EmpMapper {
    //描述一对多的拼接操作
    Set findByDeptNo(int id);
}

②xml文件书写SQL查询语句 

    
        select * from dept where deptno = #{id}
    

    
        select * from emp where state=1 and empno=#{id}
    

③实现一对多拼接

    private static void c2(){
        InputStream inputStream = Main.class.getClassLoader().getResourceAsStream("config.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        //查询部门
        DeptMapper deptMapper = sqlSession.getMapper(DeptMapper.class);
        Dept dept = deptMapper.findById2(10);
        //查询员工信息
        EmpMapper empMapper = sqlSession.getMapper(EmpMapper.class);
        Set emps = empMapper.findByDeptNo(10);
        //组装
        dept.setEmp(emps);
        System.out.println(dept);
        sqlSession.close();
    }

3、一级缓存只在一个会话间起作用，二级缓存可以解决一级缓存中不是一个会话、手动调用了清空缓存的方法、当执行更新操作（insert,update和delete）导致缓存失效等问题

一级缓存：

①创建对象，添加属性

@Data
public class Emp{
    private Integer empNo;
    private String ename;
    private String job;
    private Integer mgr;
    private String hireDate1;
    private Double sal;
    private Double comm;
    private Integer deptNo;
    public Integer state;
}

②建立接口，创建方法

public interface EmpMapper {
    Emp findById(int id);
}

③xml文件书写SQL查询语句 

    
        select * from emp where empno = #{id}
    

④实现一级缓存处理

    private static void a(){
        InputStream inputStream = Main.class.getClassLoader().getResourceAsStream("config.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        EmpMapper empMapper = sqlSession.getMapper(EmpMapper.class);
        //查询7908的员工
        Emp emp1 = empMapper.findById(7908);
        System.out.println(emp1);
        //又查询了7908的员工，这次不需要连接数据库了 使用了缓存（一级缓存）
        Emp emp2 = empMapper.findById(7908);
        System.out.println(emp2);
        sqlSession.close();
    }

二级缓存：

①在config.xml文件中开启二级缓存 ------>cacheEnabled

②让实体类序列化 ------>implements Serializable

@Data
public class Emp implements Serializable {
    private Integer empNo;
    private String ename;
    private String job;
    private Integer mgr;
    private String hireDate1;
    private Double sal;
    private Double comm;
    private Integer deptNo;
    public Integer state;
}

③在书写SQL语句的xml文件中使当前mapper参与缓存

二级缓存效果范围更大