mybatis 嵌套查询与懒加载

懒加载：对于页面有很多静态资源的情况下（比如网商购物页面），为了节省用户流量和提高页面性能，可以在用户浏览到当前资源的时候，再对资源进行请求和加载。

　　　　　　fetchType="lazy" 

mybatis多对一嵌套查询：

嵌套查询的特点就是：利用一个查询的时候发送多条sql语句；
1+n条sql语句：1条主sql,还有多条分别查询语句：

例如：我想要查询用户的详细信息包括地址，（地址在另一张表，此时表被关系表管理），我就需要嵌套查询：

1.先根据id查询用户信息：

select * from t_user where id =5

2.再根据用户信息查询用户地址的关系表得到地址表中的id：

select address_id aid from t_user_address ua where user_id =5

3.根据关系表地址id拿到地址的详细信息：

select * from t_address where id =1
select * from t_address where id =2

代码原理：

t_address 地址信息表

t_address_user地址id和用户id关系表

t_user用户表

查询的sql和结果：

 代码实现：

配置懒加载：mybatis-config.xml

 <settings>
        <setting name="lazyLoadingEnabled" value="true"/>
        <setting name="aggressiveLazyLoading" value="false"/>
    </settings>

编写sql:UserDAO.xm 1 <resultMap id="mynesting" type="user">

 <resultMap id="mynesting" type="user">
         <id property="id" column="id"></id>
         <result property="name" column="name"></result>
         <result property="sal" column="sal"></result>
         <result property="birth" column="birthday"></result>
         <collection fetchType="lazy" property="addresses" ofType="Address" column="id" javaType="ArrayList"  select="selectAddressId">
         </collection>
     </resultMap>

     <resultMap id="myua" type="ua">
         <id property="addressId" column="aid"></id>
         <collection fetchType="lazy" property="addresses" ofType="Address" column="aid" javaType="ArrayList"  select="selectAddress">
         </collection>
     </resultMap>

     <select id="ById" resultMap="mynesting">
       select * from t_user where id =#{id}
     </select>

     <select id="selectAddressId" resultMap="myua" >
         select address_id aid from t_user_address ua where user_id =#{id}
     </select>

     <select id="selectAddress" resultType="address">
       select * from t_address where id =#{id}
     </select>

UserDAO:

 package com.etc.dao;

 import com.etc.entity.User;

 import java.util.List;

 public interface UserDao {

     List<User> ById(int id);

 }

UA.java:

 package com.etc.entity;

 import java.util.List;

 public class UA {
     private int userId;
     private int addressId;
     private List<Address> addresses;

     public List<Address> getAddresses() {
         return addresses;
     }

     public void setAddresses(List<Address> addresses) {
         this.addresses = addresses;
     }

     public int getUserId() {
         return userId;
     }

     public void setUserId(int userId) {
         this.userId = userId;
     }

     public int getAddressId() {
         return addressId;
     }

     public void setAddressId(int addressId) {
         this.addressId = addressId;
     }

     @Override
     public String toString() {
         return ""+addresses;
     }
 }

UA.java

User.java:

 package com.etc.entity;

 import java.util.Date;
 import java.util.List;

 public class User {
     private int id;
     private String name;
     private double sal;
     private Date birth;
     private List<Address> addresses;

     public User() {
     }

     public User(int id, String name, double sal, Date birth, List<Address> addresses) {
         this.id = id;
         this.name = name;
         this.sal = sal;
         this.birth = birth;
         this.addresses = addresses;
     }

     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 double getSal() {
         return sal;
     }

     public void setSal(double sal) {
         this.sal = sal;
     }

     public Date getBirth() {
         return birth;
     }

     public void setBirth(Date birth) {
         this.birth = birth;
     }

     public List<Address> getAddresses() {
         return addresses;
     }

     public void setAddresses(List<Address> addresses) {
         this.addresses = addresses;
     }

     @Override
     public String toString() {
         return "User{" +
                 "id=" + id +
                 ", name='" + name + '\'' +
                 ", sal=" + sal +
                 ", birth=" + birth +
                 ", addresses=" + addresses +
                 '}';
     }
 }

User.java

address.java:

 package com.etc.entity;

 public class Address {
     private int id;
     private String name;
     private User user;

     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 User getUser() {
         return user;
     }

     public void setUser(User user) {
         this.user = user;
     }

     @Override
     public String toString() {
         return "Address{" +
                 "id=" + id +
                 ", name='" + name + '\'' +
                 ", user=" + user +
                 '}';
     }
 }

address.java

t_uesr.sql:

 /*
 Navicat MySQL Data Transfer

 Source Server         : Test
 Source Server Version : 50562
 Source Host           : localhost:3310
 Source Database       : mybatis

 Target Server Type    : MYSQL
 Target Server Version : 50562
 File Encoding         : 65001

 Date: 2019-07-25 11:43:23
 */

 SET FOREIGN_KEY_CHECKS=0;

 -- ----------------------------
 -- Table structure for t_user
 -- ----------------------------
 DROP TABLE IF EXISTS `t_user`;
 CREATE TABLE `t_user` (
   `id` int(11) NOT NULL AUTO_INCREMENT,
   `name` varchar(255) DEFAULT NULL,
   `sal` float(255,0) DEFAULT NULL,
   `birthday` date DEFAULT NULL,
   PRIMARY KEY (`id`)
 ) ENGINE=InnoDB AUTO_INCREMENT=15 DEFAULT CHARSET=utf8;

 -- ----------------------------
 -- Records of t_user
 -- ----------------------------
 INSERT INTO `t_user` VALUES ('', 'asda', '', '2019-07-22');
 INSERT INTO `t_user` VALUES ('', 'asda', '', '2019-07-22');
 INSERT INTO `t_user` VALUES ('', 'asdasdaa', '', '2019-07-22');
 INSERT INTO `t_user` VALUES ('', 'asdasdaa', '', '2019-07-22');
 INSERT INTO `t_user` VALUES ('', 'asdasdaa', '', '2019-07-22');
 INSERT INTO `t_user` VALUES ('', 'asdasdaa', '', '2019-07-22');
 INSERT INTO `t_user` VALUES ('', 'asdasdaa', '', '2019-07-22');
 INSERT INTO `t_user` VALUES ('', 'asdasdaa', '', '2019-07-22');
 INSERT INTO `t_user` VALUES ('', 'asdasdaa', '', '2019-07-23');
 INSERT INTO `t_user` VALUES ('', 'asdasdaa', '', '2019-07-23');

t_user.sql

t_address.sql:

 /*
 Navicat MySQL Data Transfer

 Source Server         : Test
 Source Server Version : 50562
 Source Host           : localhost:3310
 Source Database       : mybatis

 Target Server Type    : MYSQL
 Target Server Version : 50562
 File Encoding         : 65001

 Date: 2019-07-25 11:43:30
 */

 SET FOREIGN_KEY_CHECKS=0;

 -- ----------------------------
 -- Table structure for t_address
 -- ----------------------------
 DROP TABLE IF EXISTS `t_address`;
 CREATE TABLE `t_address` (
   `id` int(11) NOT NULL AUTO_INCREMENT,
   `name` varchar(255) DEFAULT NULL,
   PRIMARY KEY (`id`)
 ) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;

 -- ----------------------------
 -- Records of t_address
 -- ----------------------------
 INSERT INTO `t_address` VALUES ('', '这是地址');
 INSERT INTO `t_address` VALUES ('', '这是地址');
 INSERT INTO `t_address` VALUES ('', '');
 INSERT INTO `t_address` VALUES ('', '');
 INSERT INTO `t_address` VALUES ('', '');
 INSERT INTO `t_address` VALUES ('', '');

t_address.sql

t_user_address.sql:

 /*
 Navicat MySQL Data Transfer

 Source Server         : Test
 Source Server Version : 50562
 Source Host           : localhost:3310
 Source Database       : mybatis

 Target Server Type    : MYSQL
 Target Server Version : 50562
 File Encoding         : 65001

 Date: 2019-07-25 11:43:12
 */

 SET FOREIGN_KEY_CHECKS=0;

 -- ----------------------------
 -- Table structure for t_user_address
 -- ----------------------------
 DROP TABLE IF EXISTS `t_user_address`;
 CREATE TABLE `t_user_address` (
   `user_id` int(11) DEFAULT NULL,
   `address_id` int(11) DEFAULT NULL,
   UNIQUE KEY `address_unqiue` (`address_id`) USING BTREE
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;

 -- ----------------------------
 -- Records of t_user_address
 -- ----------------------------
 INSERT INTO `t_user_address` VALUES ('', '');
 INSERT INTO `t_user_address` VALUES ('', '');
 INSERT INTO `t_user_address` VALUES ('', '');
 INSERT INTO `t_user_address` VALUES ('', '');
 INSERT INTO `t_user_address` VALUES ('', '');

t_user_address.sql

懒加载：
fetchType="lazy" 

userDaoTest：

查询结果：