1.1 一對一查詢

1.1.1 概述

  關系數據庫中第一個表中的單個行只可以與第二個表中的一個行相關，且第二個表中的一個行也只可以與第一個表中的一個行相關。

1.1.2 創建實體類

public class Student { private Integer id; private String name; private Boolean age; private String sex; private StudentStatus studentStatus; // set and get}

public class StudentStatus { private Integer id; private String num; private String major; // set and get}

1.1.3 創建 DAO 接口

public class StudentStatus { private Integer id; private String num; private String major; // set and get}

1.1.4 結果映射

  resultMap 元素是 MyBatis 中最重要最強大的元素。它可以從 90% 的 JDBC ResultSets 數據提取代碼中解放出來，并在一些情形下允許進行一些 JDBC 不支持的操作。實際上，在為一些比如連接的復雜語句編寫映射代碼的時候，一份 resultMap 能夠代替實現同等功能的長達數千行的代碼。resultMap 的設計思想是，對于簡單的語句根本不需要配置顯式的結果映射，而對于復雜一點的語句只需要描述它們的關系就行了。之前已經使用過簡單映射語句了，但并沒有顯式指定 resultMap。只是簡單的使用 resultType 將所有的列映射到對象的屬性上，需要注意的是列名與屬性名一致才能映射，解決列名不匹配還是需要使用 resultMap。

<resultMap type='User'><result property='id' column='user_id' /><result property='username' column='user_name'/><result property='password' column='hashed_password'/></resultMap><select resultMap='userResultMap'>select user_id, user_name, hashed_password from some_table where id = #{id}</select>

1.1.5 配置 mapper

<?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.software.mybatis.dao.StudentDao'> <resultMap type='student'> <result property='studentStatus.id' column='st_id'/> <result property='studentStatus.major' column='major'/> <result property='studentStatus.num' column='num'/> </resultMap> <select resultMap='resMap'> select * from student s, student_status st where s.st_id = st.st_id </select></mapper>

上面這種配置會將自動將列名一致的映射到 type 指定的實體類中，該實體類中屬性類型為對象的則需要單獨拿出來映射。還可以使用 association 進行復雜的映射，我們發現未配置的屬性無法進行映射。產生這個問題的原因是 resultMap 的自動映射未打開，使用 autoMapping 設置這個屬性為 true/false，MyBatis 將會為本結果映射開啟/關閉自動映射。

<mapper namespace='com.software.mybatis.dao.StudentDao'> <resultMap type='com.software.mybatis.entity.Student'> <result property='id' column='id'/> <result property='name' column='name'/> <result property='sex' column='sex'/> <result property='age' column='age'/> <association property='studentStatus' javaType='com.software.mybatis.entity.StudentStatus'> <result property='id' column='st_id'/> <result property='major' column='major'/> <result property='num' column='num'/> </association> </resultMap> <select resultMap='resMap'> select * from student s, student_status st where s.st_id = st.st_id </select></mapper>

<?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.software.mybatis.dao.StudentDao'> <resultMap type='student' autoMapping='true'> <association property='studentStatus' resultMap='stMap' /> </resultMap> <resultMap type='StudentStatus' autoMapping='true'/> <select resultMap='resMap'> select * from student s, student_status st where s.st_id = st.st_id </select></mapper>

1.1.6 核心配置

<!DOCTYPE configuration PUBLIC '-//mybatis.org//DTD Config 3.0//EN' 'http://mybatis.org/dtd/mybatis-3-config.dtd'><configuration> <settings> <setting name='logImpl' value='STDOUT_LOGGING'/> </settings> <typeAliases> <package name='com.software.mybatis.entity'/> </typeAliases> <environments default='development'> <environment id='development'> <transactionManager type='JDBC'/> <dataSource type='POOLED'><property name='driver' value='com.mysql.jdbc.Driver'/><property name='url' value='jdbc:mysql://localhost:3306/db'/><property name='username' value='root'/><property name='password' value='root'/> </dataSource> </environment> </environments> <mappers> <mapper resource='student-mapper.xml'/> </mappers></configuration>

1.1.7 測試

/** * Created with IntelliJ IDEA. * * @author Demo_Null * @date 2020/9/3 * @description 測試類 */public class MybatisDemo { @Test public void TestA() throws IOException { // 加載核心配置文件 InputStream resourceAsStream = Resources.getResourceAsStream('mybatis.xml'); // 獲得 sqlSession 工廠對象 SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream); // 獲得 sqlSession 對象 SqlSession sqlSession = sqlSessionFactory.openSession(); List<Student> list = sqlSession.selectList('com.software.mybatis.dao.StudentDao.findAll'); System.out.println(list); }}

1.2 一對多查詢

1.2.1 概述

  一對多關系是關系數據庫中兩個表之間的一種關系，該關系中第一個表中的單個行可以與第二個表中的一個或多個行相關，但第二個表中的一個行只可以與第一個表中的一個行相關。

1.2.2 創建實體類

public class Student { private Integer sId; private String sName; private Long sAge; private String sSex; private Integer cId;// set and get}

public class Class { private Integer cId; private String cName; private String cAddr; private List<Student> students; // set and get}

1.1.3 創建 DAO 接口

/** * Created with IntelliJ IDEA. * * @author Demo_Null * @date 2020/9/3 * @description DAO 接口 */public interface ClassDao { public List<Class> findAll();}

1.1.4 配置 mapper

<?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.software.mybatis.dao.ClassDao'> <resultMap type='Class'> <result property='cId' column='c_id'/> <result property='cName' column='c_name'/> <result property='cAddr' column='c_addr'/> <collection property='students' ofType='Student'> <result property='sId' column='s_id' /> <result property='sName' column='s_name'/> <result property='sAge' column='s_age'/> <result property='sSex' column='s_sex'/> <result property='cId' column='c_id'/> </collection> </resultMap> <select resultMap='resMap'> select * from student s, class c where s.c_id = c.c_id </select></mapper>

1.1.5 測試

/** * Created with IntelliJ IDEA. * * @author Demo_Null * @date 2020/9/3 * @description 測試類 */public class MybatisDemo { @Test public void TestA() throws IOException { // 加載核心配置文件 InputStream resourceAsStream = Resources.getResourceAsStream('mybatis.xml'); // 獲得 sqlSession 工廠對象 SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream); // 獲得 sqlSession 對象 SqlSession sqlSession = sqlSessionFactory.openSession(); List<Class> list = sqlSession.selectList('com.software.mybatis.dao.ClassDao.findAll'); for (Class aClass : list) { System.out.println(aClass); } }}

1.3 多對多查詢

1.3.1 概述

  多對多關系是關系數據庫中兩個表之間的一種關系， 該關系中第一個表中的一個行可以與第二個表中的一個或多個行相關。第二個表中的一個行也可以與第一個表中的一個或多個行相關。該關系一般會借助第三方表實現。

1.3.2 創建實體類

public class Course { private Integer cId; private String cName; private List<Student> students;// set and get}

public class Student { private Integer sId; private String sName; private Long sAge; private String sSex; private List<Course> courses;// set and get}

1.3.3 創建 DAO 接口

/** * Created with IntelliJ IDEA. * * @author Demo_Null * @date 2020/9/3 * @description course DAO 接口 */public interface CourseDao { public List<Course> findAll();}

/** * Created with IntelliJ IDEA. * * @author Demo_Null * @date 2020/9/3 * @description student DAO 接口 */public interface StudentDao { public List<Student> findAll();}

1.3.4 配置 mapper

☞ student-mapper.xml

<?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.software.mybatis.dao.StudentDao'> <resultMap type='Student'> <result property='sId' column='s_id' /> <result property='sName' column='s_name'/> <result property='sAge' column='s_age'/> <result property='sSex' column='s_sex'/> <collection property='courses' ofType='Course'> <result property='cId' column='c_id'/> <result property='cName' column='c_name'/> </collection> </resultMap> <select resultMap='resMap'> select * from course c, student s, s_c sc where c.c_id = sc.c_id and s.s_id = sc.s_id </select></mapper>

☞ course-mapper.xml

<?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.software.mybatis.dao.CourseDao'> <resultMap type='Course'> <result property='cId' column='c_id'/> <result property='cName' column='c_name'/> <collection property='students' ofType='Student'> <result property='sId' column='s_id' /> <result property='sName' column='s_name'/> <result property='sAge' column='s_age'/> <result property='sSex' column='s_sex'/> </collection> </resultMap> <select resultMap='resMap'> select * from course c, student s, s_c sc where c.c_id = sc.c_id and s.s_id = sc.s_id </select></mapper>

1.3.5 測試

/** * Created with IntelliJ IDEA. * * @author Demo_Null * @date 2020/9/3 * @description 測試類 */public class MybatisDemo { @Test public void TestA() throws IOException { // 加載核心配置文件 InputStream resourceAsStream = Resources.getResourceAsStream('mybatis.xml'); // 獲得 sqlSession 工廠對象 SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream); // 獲得 sqlSession 對象 SqlSession sqlSession = sqlSessionFactory.openSession(); List<Course> courseList = sqlSession.selectList('com.software.mybatis.dao.CourseDao.findAll'); List<Student> studentList = sqlSession.selectList('com.software.mybatis.dao.StudentDao.findAll'); System.out.println('### 課程 ###'); for (Course course : courseList) { System.out.println(course); } System.out.println('### 學生 ###'); for (Student student : studentList) { System.out.println(student); } }}

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