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二手优品哪个网站做,推广平台怎么做,手册制作,网站建设课程论文一、分步查询 1.1什么时候可以用到分步查询有的时候需要我们对数据库进行关联查询，比如Employee 持有另一个Department对象的一个引用，我们希望在查询Employee 的时候把Department也一同查询出来。这时候我们可以编写关联表的SQL 语句，从而…

一、分步查询

1.1什么时候可以用到分步查询

有的时候需要我们对数据库进行关联查询，比如Employee 持有另一个Department对象的一个引用，我们希望在查询Employee 的时候把Department也一同查询出来。这时候我们可以编写关联表的SQL 语句，从而将查询出来的数据使用resultMap进行映射。

我们也可以使用其他的方式将关联的信息查询出来，MyBatis 提供了分步查询的机制，把复杂的SQL 语句简单化。

关于MyBatis关联查询可参考博文：
MyBatis使用resultMap自定义映射规则与关联映射

1.2分步查询完成JavaBean 之间关联

需求：在查询Employee信息的时候，把其对应的Department信息也一并查询出来，这里使用分步查询的方法。使用关联映射的方法可以参考上面链接内的博文。

过程分析：

1根据员工id 向员工数据库中查询信息，获得员工的信息与部门的id。

2.根据部门的id 查询部门的信息。

3.对查询的信息进行封装。

由此可见，实现分步查询的方法就是将复杂的SQL 语句拆分成简单的多个SQL 语句，在某个SQL 执行结果的基础上再进行数据查询。

1.数据库表结构：

t_employee 数据库表结构(其中d_id用于关联t_dept表中的id)：

DROP TABLE IF EXISTS `t_employee`;
CREATE TABLE `t_employee` (`id` int(11) unsigned zerofill NOT NULL AUTO_INCREMENT,`username` varchar(30) DEFAULT NULL,`gender` char(1) DEFAULT NULL,`email` varchar(20) DEFAULT NULL,`d_id` int(11) DEFAULT NULL,PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=19 DEFAULT CHARSET=utf8;
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t_dept数据库表结构：

DROP TABLE IF EXISTS `t_dept`;
CREATE TABLE `t_dept` (`id` int(11) NOT NULL AUTO_INCREMENT,`dept_name` varchar(20) DEFAULT NULL,PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;
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2.定义JavaBean

Employee类：

package com.jas.mybatis.bean;

public class Employee {
private Integer id;
private String username;
private Character gender;
private String email;
private Department department;
// 省略set、get 与toString 方法
}

Department类：

package com.jas.mybatis.bean;

public class Department {
private Integer id;
private String departmentName;
// 省略set、get 与toString 方法
}

3.定义mapper 接口

EmployeeMapper 接口：

package com.jas.mybatis.mapper;

import com.jas.mybatis.bean.Employee;

public interface EmployeeMapper {

<span class="token comment">/*** 根据员工id 获取员工信息以及对应的部门信息* * @param id* @return*/</span>
Employee <span class="token function">getEmpAndDeptById</span><span class="token punctuation">(</span>Integer id<span class="token punctuation">)</span><span class="token punctuation">;</span>

}

DepatmentMapper 接口：

package com.jas.mybatis.mapper;

import com.jas.mybatis.bean.Department;

public interface DepatmentMapper {

<span class="token comment">/*** 根据部门id 获得部门信息* * @param id* @return*/</span>
Department <span class="token function">getDeptById</span><span class="token punctuation">(</span>Integer id<span class="token punctuation">)</span><span class="token punctuation">;</span>

}

4.编写SQL 语句

DepartmentMpper.xmlSQL 映射文件：

<mapper namespace="com.jas.mybatis.mapper.DepatmentMapper"><!-- 根据部门的id 获得部门信息 --><select id="getDeptById" resultType="com.jas.mybatis.bean.Department">SELECT id, dept_name departmentName FROM t_dept WHERE id = #{id}</select>
</mapper>
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EmployeeMapper.xmlSQL 映射文件：

<mapper namespace="com.jas.mybatis.mapper.EmployeeMapper"><resultMap type="com.jas.mybatis.bean.Employee" id="BaseEmployeeResultMap"><!-- 进行数据库字段与JavaBean 属性映射 --><id column="id" property="id"/><result column="username" property="username"/><result column="gender" property="gender"/><result column="email" property="email"/>
    <span class="token comment">&lt;!--使用association 标签完成分步查询property：Employee 类中对应的属性departmentselect：另外一个映射语句的 ID，也就是DepartmentMpper.xml 中的getDeptByIdcolumn：表示数据库中的哪个字段名作为参数传给select 中的SQL 语句，如果要传递多个参数，可以把多个参数封装成map，比如：{id = d_id, ...}分析：执行流程是先根据员工的id 查询到数据库中的信息，将对应的员工信息先进行映射封装，然后根据d_id 字段值，去部门表中查询部门信息，然后再对部门信息进行映射封装，从而获得完整的员工信息。--&gt;</span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>association</span> <span class="token attr-name">property</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>department<span class="token punctuation">"</span></span> <span class="token attr-name">column</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>d_id<span class="token punctuation">"</span></span><span class="token attr-name">select</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>com.jas.mybatis.mapper.DepatmentMapper.getDeptById<span class="token punctuation">"</span></span><span class="token punctuation">&gt;</span></span><span class="token comment">&lt;!--将数据库中的部门信息映射到Employee 类中的部门信息属性中--&gt;</span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>id</span> <span class="token attr-name">column</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>d_id<span class="token punctuation">"</span></span> <span class="token attr-name">property</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>id<span class="token punctuation">"</span></span><span class="token punctuation">/&gt;</span></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>result</span> <span class="token attr-name">column</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>dept_name<span class="token punctuation">"</span></span> <span class="token attr-name">property</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>departmentName<span class="token punctuation">"</span></span><span class="token punctuation">/&gt;</span></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>association</span><span class="token punctuation">&gt;</span></span>
<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>resultMap</span><span class="token punctuation">&gt;</span></span><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>select</span> <span class="token attr-name">id</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>getEmpAndDeptById<span class="token punctuation">"</span></span> <span class="token attr-name">resultMap</span><span class="token attr-value"><span class="token punctuation">=</span><span class="token punctuation">"</span>BaseEmployeeResultMap<span class="token punctuation">"</span></span><span class="token punctuation">&gt;</span></span>SELECT * FROM t_employee WHERE id = #{id}
<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>select</span><span class="token punctuation">&gt;</span></span>

</mapper>

5.测试代码：

    private SqlSession getSqlSession() throws IOException {String resource = "mybatis-config.xml";InputStream is = Resources.getResourceAsStream(resource);
    SqlSessionFactory sqlSessionFactory <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">SqlSessionFactoryBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">build</span><span class="token punctuation">(</span>is<span class="token punctuation">)</span><span class="token punctuation">;</span>SqlSession sqlSession <span class="token operator">=</span> sqlSessionFactory<span class="token punctuation">.</span><span class="token function">openSession</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token keyword">return</span> sqlSession<span class="token punctuation">;</span>
<span class="token punctuation">}</span><span class="token annotation punctuation">@Test</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">resultMapTest01</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> IOException <span class="token punctuation">{</span>SqlSession sqlSession <span class="token operator">=</span> <span class="token function">getSqlSession</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>EmployeeMapper employeeMapper <span class="token operator">=</span> sqlSession<span class="token punctuation">.</span><span class="token function">getMapper</span><span class="token punctuation">(</span>EmployeeMapper<span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span>Employee employee <span class="token operator">=</span> employeeMapper<span class="token punctuation">.</span><span class="token function">getEmpAndDeptById</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>employee<span class="token punctuation">)</span><span class="token punctuation">;</span>sqlSession<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

6.查看结果：

这里写图片描述

从上面的执行结果可以看出在进行Employee查询结果信息封装的时候，向数据库发了两条SQL 语句，分别获得员工信息与部门信息。通过把复杂的关联SQL 语句进行分步，最终完成了分步查询。

一般情况下Department getDeptById(Integer id);方法在实际的开发过程中是必须要定义的一个方法，这样在进行关联查询的时候，我们只要使用现成的方法即可，不需要再定义复杂的关联SQL 语句。分步查询还有一个好处就是可以实现延迟加载，这个延迟加载功能在MyBatis 中可以说是一个非常重要的功能。

下面我们就来看看分步查询是如何完成延迟加载的。

二、延迟加载

2.1什么是延迟加载

延迟加载也可以理解为按需加载。即在你需要的时候才进行的加载方式，才会向数据库发送SQL 语句，不需要的时候不会向数据库中发送SQL。这样做可以用于减轻数据库的压力。

比如上面的分步查询，我们通过分步查询分别查询出了员工信息与部门信息，并把这些信息封装成了一个Employee对象。如果我们并不需要查询员工的部门信息，在查询员工信息的时候，由于分步查询了部门信息，所以这时候部门信息也会被查出来，但是查询出来的部门信息是不需要的。本来向数据库发送一条SQL 语句就可以了，这时候发送了两条。显然这样做不是好的解决办法，这时候我们就可以使用延迟加载。

2.2实现延迟加载

其实在MyBais 中实现延迟加载的功能是很容易的，只需要在全局的配置文件中进行对应的setting设置即可。

    <settings><!-- 延迟加载的全局开关。当开启时，所有关联对象都会延迟加载，默认为false  --><setting name="lazyLoadingEnabled" value="true"/><!-- 设置属性按需加载 --><setting name="aggressiveLazyLoading" value="false"/></settings>
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你有没有觉得还需要点什么，其实只要配置了上面的两个属性，延迟加载就已经开启了。

2.3测试：

如果我们只要获得员工的姓名：

	// 省略了 getSqlSession() 方法@Testpublic void resultMapTest01() throws IOException {
    SqlSession sqlSession <span class="token operator">=</span> <span class="token function">getSqlSession</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>EmployeeMapper employeeMapper <span class="token operator">=</span> sqlSession<span class="token punctuation">.</span><span class="token function">getMapper</span><span class="token punctuation">(</span>EmployeeMapper<span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span>Employee employee <span class="token operator">=</span> employeeMapper<span class="token punctuation">.</span><span class="token function">getEmpAndDeptById</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>employee<span class="token punctuation">.</span><span class="token function">getUsername</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>sqlSession<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>      
<span class="token punctuation">}</span>

执行结果：
这里写图片描述
开启延迟加载后只发送了一条SQL 语句。

下面我们来看看在没有开启延迟加载的情况下获得员工的姓名：
这里写图片描述
发现即使你不想要获得员工的部门信息，由于执行的是分步查询，所以还是会发送两条SQL 语句，这显然不是我们想要的。

如果我们想要获得员工的部门信息：

    @Testpublic void resultMapTest01() throws IOException {
    SqlSession sqlSession <span class="token operator">=</span> <span class="token function">getSqlSession</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>EmployeeMapper employeeMapper <span class="token operator">=</span> sqlSession<span class="token punctuation">.</span><span class="token function">getMapper</span><span class="token punctuation">(</span>EmployeeMapper<span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span>Employee employee <span class="token operator">=</span> employeeMapper<span class="token punctuation">.</span><span class="token function">getEmpAndDeptById</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>employee<span class="token punctuation">.</span><span class="token function">getDepartment</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>sqlSession<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>