集合嵌套查询和集合嵌套结果的区别
嵌套查询是多条sql语句分开写并配置，嵌套结果是一条sql语句关联查询并配置，实质效果是一样的。嵌套语句的查询会导致数据库访问次数不定，进而有可能影响到性能。
1.创建2张表，建立主外键关系
2.建立实体类
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package com.yw.test06; public class Class { private int id; private String name; 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; } @Override public String toString() { return "Class [id=" + id + ", name=" + name + "]" ; } }
package com.yw.test06; public class Student { private int id; private String name; private int age; private Class c;
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 Class getC() { return c; } public void setC(Class c) { this .c = c; } @Override public String toString() { return "Student [id=" + id + ", name=" + name + ", age=" + age + ", c=" + c + "]" ; } } |
3.修改配置文件
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<? 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 = "config.properties" > </ properties > < typeAliases > <!-- <typeAlias type="com.yw.test06.StudentMapper" alias="Student" /> <typeAlias type="com.yw.test06.ClassMapper" alias="Class" /> --> < package name = "com.yw.test06" /> </ typeAliases > < 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="org/mybatis/example/BlogMapper.xml"/> --> < mapper resource = "com/yw/test06/StudentMapper.xml" /> </ mappers >
</ configuration > |
4.建立映射文件
1）嵌套查询
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<? 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.yw.test07.StudentMapper" >
< select id = "selectStudent" resultMap = "studentResult" > SELECT * FROM student s WHERE s.ID = #{id} </ select > < resultMap type = "Student" id = "studentResult" > < association property = "c" column = "c_id" javaType = "Class" select = "selectClass" /> </ resultMap >
< select id = "selectClass" resultType = "Class" > SELECT * FROM class WHERE ID= #{id} </ select >
</ mapper > |
2）嵌套结果
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<? 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.yw.test06.StudentMapper" > < resultMap id = "studentResult" type = "com.yw.test06.Student" > < id property = "id" column = "id" /> < result property = "name" column = "name" /> < result property = "age" column = "age" /> < association property = "c" resultMap = "classResult" javaType = "Class" ></ association >
</ resultMap > < resultMap type = "com.yw.test06.Class" id = "classResult" > < id property = "id" column = "id" /> < result property = "name" column = "name" /> </ resultMap >
< select id = "selectStudent" resultMap = "studentResult" > SELECT s.id,c.id,s.name,s.age,c.name from student s left join class c on c.id=s.c_id where s.id=#{id} </ select > </ mapper > |
5.创建测试类
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package com.yw.test06; import java.io.IOException; import java.io.InputStream; import org.apache.ibatis.io.Resources; import org.apache.ibatis.session.SqlSession; import org.apache.ibatis.session.SqlSessionFactory; import org.apache.ibatis.session.SqlSessionFactoryBuilder; public class Test01 { public static void main(String[] args) throws IOException {
String resource = "com/yw/test06/mybatis-config.xml" ; InputStream inputStream = Resources.getResourceAsStream(resource); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession session = sqlSessionFactory.openSession( false ); try { Student user = (Student) session.selectOne( "com.yw.test06.StudentMapper.selectStudent" , 1 ); System.out.println(user);
} finally { session.close(); } } } |
MyBatis 嵌套查询解析
Mybatis表现关联关系比hibernate简单，没有分那么细致one-to-many、many-to-one、one-to-one。而是只有两种association（一）、collection（多），表现很简洁。下面通过一个实例，来展示一下Mybatis对于常见的一对多和多对一关系复杂映射是怎样处理的。
以最简单的用户表订单表这个最简单的一对多做示例
对应的JavaBean
User:
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public class User { private int id; private String name; private Double age; private List<User_orders> orders; // get set 省 } |
User_orders:
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public class User_orders { private int id; private String name; // get set 省 } |
对应的数据库
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mysql> desc user ; + -------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | + -------+-------------+------+-----+---------+----------------+ | id | int (11) | NO | PRI | NULL | auto_increment | | name | varchar (20) | NO | | NULL | | | age | double | YES | | NULL | | + -------+-------------+------+-----+---------+----------------+ 3 rows in set (0.00 sec) mysql> desc user_orders; + ---------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | + ---------+-------------+------+-----+---------+----------------+ | id | int (11) | NO | PRI | NULL | auto_increment | | name | varchar (20) | NO | | NULL | | | user_id | int (5) | YES | MUL | NULL | | + ---------+-------------+------+-----+---------+----------------+ 3 rows in set (0.00 sec) |
现在查询一个user的id查询出所有信息.如果不考虑关联查询，我们会先根据user的id在user表中查询出name,age然后设置给User类的时候，再根据该user的id在user_orders表中查询出所有订单并设置给User类。这样的话，在底层最起码调用两次查询语句，得到需要的信息，然后再组装User对象。
嵌套语句查询
mybatis提供了一种机制，叫做嵌套语句查询，可以大大简化上述的操作，加入配置及代码如下：
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<resultMap type= "domain.User" id= "user" > <id column = "id" property= "id" /> <result column = "age" property= "age" /> <collection column = "id" property= "orders" ofType= "domain.User_orders" select = "selectOrderByUser" > <id column = "id" property= "id" /> <result column = "name" property= "name" /> </collection> </resultMap> < select id= "selectOrderByUser" parameterType= "integer" resultType= "domain.User_orders" > select id, name from user_orders where user_id = #{id} </ select > < select id= "findById" resultMap= "user" parameterType= "integer" > select * from user where id = #{id} </ select > |
测试(可以成功查询到所有信息):
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String config = "sqlMapConfig.xml" ; InputStream inputStream = Resources.getResourceAsStream(config); sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); SqlSession session = sqlSessionFactory.openSession(); // 执行在bean配置文件中定义的sql语句 User user = session.selectOne( "UserMapper.findById" , 1 ); //一句即可获取到复杂的User对象。 System.out.println(user); session测试数据mit(); session.close(); |
嵌套语句查询的原理
在上面的代码中，Mybatis会执行以下流程：
1.先执行 findById 对应的语句从User表里获取到ResultSet结果集；
2.取出ResultSet下一条有效记录，然后根据resultMap定义的映射规格，通过这条记录的数据来构建对应的一个User 对象。
当要对User中的orders属性进行赋值的时候，发现有一个关联的查询，此时Mybatis会先执行这个select查询语句，得到返回的结果，将结果设置到user的orders属性上这种关联的嵌套查询，有一个非常好的作用就是：可以重用select语句，通过简单的select语句之间的组合来构造复杂的对象。想如上的两个select完全可以独立使用。
嵌套查询的多对一
上面的关联查询查询其实是对于一对多的查询，即从user中查出user_order的信息。
现在从user_order中查user的信息.
在User_order表中增加字段user:
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public class User_orders { private int id; private String name; private User user; //xxx } |
配置select:
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< resultMap type = "domain.User_orders" id = "user_order" > < id column = "id" property = "id" /> < result column = "name" property = "name" /> < association property = "user" column = "user_id" javaType = "domain.User" select = "selectUserByOrderId" > < id column = "id" property = "id" /> < result column = "age" property = "age" /> </ association > </ resultMap > < select id = "selectUserByOrderId" parameterType = "INTEGER" resultType = "domain.User" > select id,age from user where id = #{id} </ select > < select id = "findOne" resultMap = "user_order" parameterType = "integer" > select * from user_orders where id=#{id} </ select > |
测试:
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SqlSession session = sqlSessionFactory.openSession(); // 执行在bean配置文件中定义的sql语句 User_orders user_orders= session.selectOne( "User_ordersMapper.findOne" , 1); System. out .println(user_orders); //查询到了user_order对应的 user 的信息 session. commit (); session. close (); |
嵌套查询的N+1问题
尽管嵌套查询大量的简化了存在关联关系的查询，但它的弊端也比较明显：即所谓的N+1问题。关联的嵌套查询显示得到一个结果集，然后根据这个结果集的每一条记录进行关联查询。
现在假设嵌套查询就一个（即resultMap 内部就一个association标签），现查询的结果集返回条数为N，那么关联查询语句将会被执行N次，加上自身返回结果集查询1次，共需要访问数据库N+1次。如果N比较大的话，这样的数据库访问消耗是非常大的！所以使用这种嵌套语句查询的使用者一定要考虑慎重考虑，确保N值不会很大。
以上面一对多(根据user的id查询order)的例子为例，select 语句本身会返回user条数为1 的结果集，由于它存在有1条关联的语句查询，它需要共访问数据库 1*（1+1）=2次数据库。
嵌套结果查询
嵌套语句的查询会导致数据库访问次数不定，进而有可能影响到性能。Mybatis还支持一种嵌套结果的查询：即对于一对多，多对多，多对一的情况的查询，Mybatis通过联合查询，将结果从数据库内一次性查出来，然后根据其一对多，多对一，多对多的关系和ResultMap中的配置，进行结果的转换，构建需要的对象。
重新定义User的结果映射 resultMap
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< resultMap type = "domain.User" id = "user_auto" > < id column = "id" property = "id" /> < result column = "age" property = "age" /> < collection column = "id" property = "orders" ofType = "domain.User_orders" > < id column = "order_id" property = "id" /> < result column = "name" property = "name" /> </ collection > </ resultMap > |
对应的sql语句如下：
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< select id= "findAuth" resultMap= "user_auto" > select u.id,u.age,o.id as order_id ,o. name ,o.user_id as user_id from user u left outer join user_orders o on o.user_id = u.id </ select > |
嵌套结果查询的执行步骤
1.根据表的对应关系，进行join操作，获取到结果集；
根据结果集的信息和user 的resultMap定义信息，对返回的结果集在内存中进行组装、赋值，构造User；
返回构造出来的结果List 结果。
对于关联的结果查询，如果是多对一的关系，则通过形如 <association property="user" column="user_id" javaType="domain.User" > 进行配置，Mybatis会通过column属性对应的user_id 值去从内存中取数据，并且封装成User_order对象；
如果是一对多的关系，就如User和User_order之间的关系，通过形如 <collection column="id" property="orders" ofType="domain.User_orders">进行配置，MyBatis通过 id去内存中取User_orders对象，封装成List；
对于关联结果的查询，只需要查询数据库一次，然后对结果的整合和组装全部放在了内存中。
以上是通过查询User表所有信息来演示了一对多和多对一的映射对象处理。希望能给大家一个参考，也希望大家多多支持。
原文链接：https://blog.csdn.net/qq_36468810/article/details/109356533
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