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Student中包含Address

package com.skymr.mybatis.model;public class Student {	private int id;		private String name;		private int age;		private Address address;		/**	 * 必须要有无参构造器，有参构造器可有可无（至少我测试时是这样）	 * 如果没有无参构造器，只有有参构造器，会报错	 */	public Student() {	}	public Student(String name, int age) {		this.name = name;		this.age = age;	}	public int getId() {		return id;	}	public void setId(int id) {		this.id = id;	}	public String getName() {		return name;	}	public void setName(String name) {		this.name = name;	}	public int getAge() {		return age;	}	public void setAge(int age) {		this.age = age;	}		public Address getAddress() {		return address;	}	public void setAddress(Address address) {		this.address = address;	}	public String toString(){		return "["+name+","+age+","+address+"]";	}	}

package com.skymr.mybatis.model;public class Address {	//省	private String province;	//市	private String city;	//区	private String region;		private int id;	public String getProvince() {		return province;	}	public void setProvince(String province) {		this.province = province;	}	public String getCity() {		return city;	}	public void setCity(String city) {		this.city = city;	}	public String getRegion() {		return region;	}	public void setRegion(String region) {		this.region = region;	}	public int getId() {		return id;	}	public void setId(int id) {		this.id = id;	}		public String toString(){		return "[" + id+","+this.province+","+this.city+","+this.region+"]";	}}

package com.skymr.mybatis.mappers;import java.util.List;import com.skymr.mybatis.model.Student;public interface StudentMapper {	public Student getStudent(int id);		public List
   
     getAllStudents();	//取得所有学生，带地址	public List
    
      getAllStudentsWithAddr();	//取得学生，带地址	public Student getStudentWithAddr(int id);}
    
   

方式一：对象集联

   			select * from mybatis_Student t1, mybatis_address t2 where t1.address_id=t2.id		
   		
    		
    		
    		
    		
    		
    		
    	
   				select * from mybatis_Student t1 left join mybatis_address t2 on t1.address_id=t2.id and t1.id=#{id}	

这种方式不太好，重用性差．

方式二：

   			select * from mybatis_Student t1, mybatis_address t2 where t1.address_id=t2.id		
   		
    		
    		
    		
    	
   	
   	
   		
    		
    		
    		
    	
   			select * from mybatis_Student t1 left join mybatis_address t2 on t1.address_id=t2.id and t1.id=#{id}	

将Address独立出来，然后使用association关联到Address的resultMap

方式三：

方式四：推荐的方式

这种方式还是主要是应用association标签

首先，为Address类加入AddressMapper类与AddressMapper.xml

package com.skymr.mybatis.mappers;import com.skymr.mybatis.model.Address;public interface AddressMapper {	public Address getAddress(int id);}

   
   	
    		select * from mybatis_address where id=#{id}	
   

其次，修改StudentMapper.xml

   			select * from mybatis_Student		
   		
    		
    		
    		
    		
    	
   			select * from mybatis_Student where id=#{id}	

使用了association标签，定义column与select属性

column:　传入mybatis_Student表的外键address_id.

select:　调用AddressMapper的getAddress方法

推演过程：查询一条student数据，再根据address_id查询address表的一条数据．

思考：不知道可不可以这样想，如果查询1000条student，再分别查address表的1000条数据，这样查询次数就是1001次了，不影响性能吗？

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