一、一對一關聯
1.1、提出需求
根據班級id查詢班級信息(帶老師的信息)
1.2、創建表和數據
創建一張教師表和班級表,這裏我們假設一個老師只負責教一個班,那麼老師和班級之間的關係就是一種一對一的關係。
- CREATE TABLE teacher(
- t_id INT PRIMARY KEY AUTO_INCREMENT,
- t_name VARCHAR(20)
- );
- CREATE TABLE class(
- c_id INT PRIMARY KEY AUTO_INCREMENT,
- c_name VARCHAR(20),
- teacher_id INT
- );
- ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES teacher(t_id);
-
- INSERT INTO teacher(t_name) VALUES('teacher1');
- INSERT INTO teacher(t_name) VALUES('teacher2');
-
- INSERT INTO class(c_name, teacher_id) VALUES('class_a', 1);
- INSERT INTO class(c_name, teacher_id) VALUES('class_b', 2);
表之間的關係如下:
1.3、定義實體類
1、Teacher類,Teacher類是teacher表對應的實體類。
- package me.gacl.domain;
-
- /**
- * @author gacl
- * 定義teacher表對應的實體類
- */
- public class Teacher {
-
- //定義實體類的屬性,與teacher表中的字段對應
- private int id; //id===>t_id
- private String name; //name===>t_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 "Teacher [id=" + id + ", name=" + name + "]";
- }
- }
2、Classes類,Classes類是class表對應的實體類
- package me.gacl.domain;
-
- /**
- * @author gacl
- * 定義class表對應的實體類
- */
- public class Classes {
-
- //定義實體類的屬性,與class表中的字段對應
- private int id; //id===>c_id
- private String name; //name===>c_name
-
- /**
- * class表中有一個teacher_id字段,所以在Classes類中定義一個teacher屬性,
- * 用於維護teacher和class之間的一對一關係,通過這個teacher屬性就可以知道這個班級是由哪個老師負責的
- */
- private Teacher teacher;
-
- 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 Teacher getTeacher() {
- return teacher;
- }
-
- public void setTeacher(Teacher teacher) {
- this.teacher = teacher;
- }
-
- @Override
- public String toString() {
- return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher+ "]";
- }
- }
1.4、定義sql映射文件classMapper.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">
- <!-- 爲這個mapper指定一個唯一的namespace,namespace的值習慣上設置成包名+sql映射文件名,這樣就能夠保證namespace的值是唯一的
- 例如namespace="me.gacl.mapping.classMapper"就是me.gacl.mapping(包名)+classMapper(classMapper.xml文件去除後綴)
- -->
- <mapper namespace="me.gacl.mapping.classMapper">
-
- <!--
- 根據班級id查詢班級信息(帶老師的信息)
- ##1. 聯表查詢
- SELECT * FROM class c,teacher t WHERE c.teacher_id=t.t_id AND c.c_id=1;
-
- ##2. 執行兩次查詢
- SELECT * FROM class WHERE c_id=1; //teacher_id=1
- SELECT * FROM teacher WHERE t_id=1;//使用上面得到的teacher_id
- -->
-
- <!--
- 方式一:嵌套結果:使用嵌套結果映射來處理重複的聯合結果的子集
- 封裝聯表查詢的數據(去除重複的數據)
- select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=1
- -->
- <select id="getClass" parameterType="int" resultMap="ClassResultMap">
- select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id}
- </select>
- <!-- 使用resultMap映射實體類和字段之間的一一對應關係 -->
- <resultMap type="me.gacl.domain.Classes" id="ClassResultMap">
- <id property="id" column="c_id"/>
- <result property="name" column="c_name"/>
- <association property="teacher" javaType="me.gacl.domain.Teacher">
- <id property="id" column="t_id"/>
- <result property="name" column="t_name"/>
- </association>
- </resultMap>
-
- <!--
- 方式二:嵌套查詢:通過執行另外一個SQL映射語句來返回預期的複雜類型
- SELECT * FROM class WHERE c_id=1;
- SELECT * FROM teacher WHERE t_id=1 //1 是上一個查詢得到的teacher_id的值
- -->
- <select id="getClass2" parameterType="int" resultMap="ClassResultMap2">
- select * from class where c_id=#{id}
- </select>
- <!-- 使用resultMap映射實體類和字段之間的一一對應關係 -->
- <resultMap type="me.gacl.domain.Classes" id="ClassResultMap2">
- <id property="id" column="c_id"/>
- <result property="name" column="c_name"/>
- <association property="teacher" column="teacher_id" select="getTeacher"/>
- </resultMap>
-
- <select id="getTeacher" parameterType="int" resultType="me.gacl.domain.Teacher">
- SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
- </select>
-
- </mapper>
在conf.xml文件中註冊classMapper.xml
<mappers> <!-- 註冊classMapper.xml文件, classMapper.xml位於me.gacl.mapping這個包下,所以resource寫成me/gacl/mapping/classMapper.xml--> <mapper resource="me/gacl/mapping/classMapper.xml"/> </mappers>
1.5、編寫單元測試代碼
- package me.gacl.test;
-
- import me.gacl.domain.Classes;
- import me.gacl.util.MyBatisUtil;
- import org.apache.ibatis.session.SqlSession;
- import org.junit.Test;
-
- public class Test3 {
-
- @Test
- public void testGetClass(){
- SqlSession sqlSession = MyBatisUtil.getSqlSession();
- /**
- * 映射sql的標識字符串,
- * me.gacl.mapping.classMapper是classMapper.xml文件中mapper標籤的namespace屬性的值,
- * getClass是select標籤的id屬性值,通過select標籤的id屬性值就可以找到要執行的SQL
- */
- String statement = "me.gacl.mapping.classMapper.getClass";//映射sql的標識字符串
- //執行查詢操作,將查詢結果自動封裝成Classes對象返回
- Classes clazz = sqlSession.selectOne(statement,1);//查詢class表中id爲1的記錄
- //使用SqlSession執行完SQL之後需要關閉SqlSession
- sqlSession.close();
- System.out.println(clazz);//打印結果:Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1]]
- }
-
- @Test
- public void testGetClass2(){
- SqlSession sqlSession = MyBatisUtil.getSqlSession();
- /**
- * 映射sql的標識字符串,
- * me.gacl.mapping.classMapper是classMapper.xml文件中mapper標籤的namespace屬性的值,
- * getClass2是select標籤的id屬性值,通過select標籤的id屬性值就可以找到要執行的SQL
- */
- String statement = "me.gacl.mapping.classMapper.getClass2";//映射sql的標識字符串
- //執行查詢操作,將查詢結果自動封裝成Classes對象返回
- Classes clazz = sqlSession.selectOne(statement,1);//查詢class表中id爲1的記錄
- //使用SqlSession執行完SQL之後需要關閉SqlSession
- sqlSession.close();
- System.out.println(clazz);//打印結果:Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1]]
- }
- }
1.6、MyBatis一對一關聯查詢總結
MyBatis中使用association標籤來解決一對一的關聯查詢,association標籤可用的屬性如下:
- property:對象屬性的名稱
- javaType:對象屬性的類型
- column:所對應的外鍵字段名稱
- select:使用另一個查詢封裝的結果
二、一對多關聯
2.1、提出需求
根據classId查詢對應的班級信息,包括學生,老師
2.2、創建表和數據
在上面的一對一關聯查詢演示中,我們已經創建了班級表和教師表,因此這裏再創建一張學生表
CREATE TABLE student( s_id INT PRIMARY KEY AUTO_INCREMENT, s_name VARCHAR(20), class_id INT ); INSERT INTO student(s_name, class_id) VALUES('student_A', 1); INSERT INTO student(s_name, class_id) VALUES('student_B', 1); INSERT INTO student(s_name, class_id) VALUES('student_C', 1); INSERT INTO student(s_name, class_id) VALUES('student_D', 2); INSERT INTO student(s_name, class_id) VALUES('student_E', 2); INSERT INTO student(s_name, class_id) VALUES('student_F', 2);
2.3、定義實體類
1、Student類
- package me.gacl.domain;
-
- /**
- * @author gacl
- * 定義student表所對應的實體類
- */
- public class Student {
-
- //定義屬性,和student表中的字段對應
- private int id; //id===>s_id
- private String name; //name===>s_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 "Student [id=" + id + ", name=" + name + "]";
- }
- }
2、修改Classes類,添加一個List<Student> students屬性,使用一個List<Student>集合屬性表示班級擁有的學生,如下:
- package me.gacl.domain;
-
- import java.util.List;
-
- /**
- * @author gacl
- * 定義class表對應的實體類
- */
- public class Classes {
-
- //定義實體類的屬性,與class表中的字段對應
- private int id; //id===>c_id
- private String name; //name===>c_name
-
- /**
- * class表中有一個teacher_id字段,所以在Classes類中定義一個teacher屬性,
- * 用於維護teacher和class之間的一對一關係,通過這個teacher屬性就可以知道這個班級是由哪個老師負責的
- */
- private Teacher teacher;
- //使用一個List<Student>集合屬性表示班級擁有的學生
- private List<Student> students;
-
- 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 Teacher getTeacher() {
- return teacher;
- }
-
- public void setTeacher(Teacher teacher) {
- this.teacher = teacher;
- }
-
- public List<Student> getStudents() {
- return students;
- }
-
- public void setStudents(List<Student> students) {
- this.students = students;
- }
-
- @Override
- public String toString() {
- return "Classes [id=" + id + ", name=" + name + ", teacher=" + teacher
- + ", students=" + students + "]";
- }
- }
2.4、修改sql映射文件classMapper.xml
添加如下的SQL映射信息
- <!--
- 根據classId查詢對應的班級信息,包括學生,老師
- -->
- <!--
- 方式一: 嵌套結果: 使用嵌套結果映射來處理重複的聯合結果的子集
- SELECT * FROM class c, teacher t,student s WHERE c.teacher_id=t.t_id AND c.C_id=s.class_id AND c.c_id=1
- -->
- <select id="getClass3" parameterType="int" resultMap="ClassResultMap3">
- select * from class c, teacher t,student s where c.teacher_id=t.t_id and c.C_id=s.class_id and c.c_id=#{id}
- </select>
- <resultMap type="me.gacl.domain.Classes" id="ClassResultMap3">
- <id property="id" column="c_id"/>
- <result property="name" column="c_name"/>
- <association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher">
- <id property="id" column="t_id"/>
- <result property="name" column="t_name"/>
- </association>
- <!-- ofType指定students集合中的對象類型 -->
- <collection property="students" ofType="me.gacl.domain.Student">
- <id property="id" column="s_id"/>
- <result property="name" column="s_name"/>
- </collection>
- </resultMap>
-
- <!--
- 方式二:嵌套查詢:通過執行另外一個SQL映射語句來返回預期的複雜類型
- SELECT * FROM class WHERE c_id=1;
- SELECT * FROM teacher WHERE t_id=1 //1 是上一個查詢得到的teacher_id的值
- SELECT * FROM student WHERE class_id=1 //1是第一個查詢得到的c_id字段的值
- -->
- <select id="getClass4" parameterType="int" resultMap="ClassResultMap4">
- select * from class where c_id=#{id}
- </select>
- <resultMap type="me.gacl.domain.Classes" id="ClassResultMap4">
- <id property="id" column="c_id"/>
- <result property="name" column="c_name"/>
- <association property="teacher" column="teacher_id" javaType="me.gacl.domain.Teacher" select="getTeacher2"></association>
- <collection property="students" ofType="me.gacl.domain.Student" column="c_id" select="getStudent"></collection>
- </resultMap>
-
- <select id="getTeacher2" parameterType="int" resultType="me.gacl.domain.Teacher">
- SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
- </select>
-
- <select id="getStudent" parameterType="int" resultType="me.gacl.domain.Student">
- SELECT s_id id, s_name name FROM student WHERE class_id=#{id}
- </select>
2.5、編寫單元測試代碼
- package me.gacl.test;
-
- import me.gacl.domain.Classes;
- import me.gacl.util.MyBatisUtil;
- import org.apache.ibatis.session.SqlSession;
- import org.junit.Test;
-
- public class Test4 {
-
- @Test
- public void testGetClass3(){
- SqlSession sqlSession = MyBatisUtil.getSqlSession();
- /**
- * 映射sql的標識字符串,
- * me.gacl.mapping.classMapper是classMapper.xml文件中mapper標籤的namespace屬性的值,
- * getClass3是select標籤的id屬性值,通過select標籤的id屬性值就可以找到要執行的SQL
- */
- String statement = "me.gacl.mapping.classMapper.getClass3";//映射sql的標識字符串
- //執行查詢操作,將查詢結果自動封裝成Classes對象返回
- Classes clazz = sqlSession.selectOne(statement,1);//查詢class表中id爲1的記錄
- //使用SqlSession執行完SQL之後需要關閉SqlSession
- sqlSession.close();
- //打印結果:Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1], students=[Student [id=1, name=student_A], Student [id=2, name=student_B], Student [id=3, name=student_C]]]
- System.out.println(clazz);
- }
-
- @Test
- public void testGetClass4(){
- SqlSession sqlSession = MyBatisUtil.getSqlSession();
- /**
- * 映射sql的標識字符串,
- * me.gacl.mapping.classMapper是classMapper.xml文件中mapper標籤的namespace屬性的值,
- * getClass4是select標籤的id屬性值,通過select標籤的id屬性值就可以找到要執行的SQL
- */
- String statement = "me.gacl.mapping.classMapper.getClass4";//映射sql的標識字符串
- //執行查詢操作,將查詢結果自動封裝成Classes對象返回
- Classes clazz = sqlSession.selectOne(statement,1);//查詢class表中id爲1的記錄
- //使用SqlSession執行完SQL之後需要關閉SqlSession
- sqlSession.close();
- //打印結果:Classes [id=1, name=class_a, teacher=Teacher [id=1, name=teacher1], students=[Student [id=1, name=student_A], Student [id=2, name=student_B], Student [id=3, name=student_C]]]
- System.out.println(clazz);
- }
- }
2.6、MyBatis一對多關聯查詢總結
MyBatis中使用collection標籤來解決一對多的關聯查詢,ofType屬性指定集合中元素的對象類型。