mybatis 建立臨時表_MyBatis的多表查詢
阿新 • • 發佈：2021-01-30
技術標籤：mybatis 建立臨時表
多對一
結果巢狀
1：建立mybatis配置檔案（mybatis-config.xml）
<?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="db.properties"/> <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="cn/sxt/entity/student-mapper.xml" /> </mappers> </configuration>
- 這裡使用了properties檔案，可以不使用
driver=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/jdbc
username=root
password=12342：建立工具類（MybatisUtils.java）用於獲取SqlSession物件
public class MybatisUtil { public static SqlSessionFactory getSqlSessionFactory() throws IOException{ String resource = "mybatis-config.xml"; InputStream inputStream = Resources.getResourceAsStream(resource); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); return sqlSessionFactory; } public static SqlSession getSqlSession() throws IOException{ SqlSession sqlsession = getSqlSessionFactory().openSession(); return sqlsession; } }
- String resource ="mybatis-config.xml" 為“包名+檔名”，此處mybatis配置檔案直接放在src下，省略包名
3：建立實體類物件
public class Student { private int id; private String name; private int tid; 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 int getTid() { return tid; } public void setTid(int tid) { this.tid = tid; } public Teacher getTeacher() { return teacher; } public void setTeacher(Teacher teacher) { this.teacher = teacher; } }
- 從Student入手的多對一查詢，需要在Student中新增一個Teacher類屬性用於存放teacher表
public class Teacher {
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;
}
}4：建立mapper對映檔案用於書寫SQL語句
<?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="cn.sxt.entity.student-mapper">
<select id="getStudentAndTeacher" resultMap="getStudentAndTeacherMap">
select s.id,s.name,s.tid,t.id,t.name from student s,teacher t where s.tid=t.id
</select>
<resultMap type="cn.sxt.entity.Student" id="getStudentAndTeacherMap">
<id column="id" property="id"/>
<result column="name" property="name"/>
<result column="tid" property="tid"/>
<!-- association為關聯物件，在進行多表查詢且相應返回型別的實體類中擁有其他類時使用。property表示資料庫中的表，JavaType表示對應的實體類 -->
<association property="teacher" javaType="cn.sxt.entity.Teacher">
<id column="id" property="id"/>
<result column="name" property="name"/>
</association>
</resultMap>
</mapper>- 從Student入手的多對一查詢，需要在Student實體類中新增一個Teacher類屬性，同時在mapper對映檔案中使用resultMap標籤，在其內使用association標籤。association表示關聯物件，其內的property屬性表示資料庫中的表，JavaType表示對應的實體類
- 資料庫中的兩個表的列名是相同的，為避免混亂，在查詢時最好使用別名，如下（不使用也可，如上）
<?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="cn.sxt.entity.student-mapper">
<select id="getStudentAndTeacher" resultMap="getStudentAndTeacherMap">
select s.id sid,s.name sname,s.tid stid,t.id tid,t.name tname from student s,teacher t where s.tid=t.id
</select>
<resultMap type="cn.sxt.entity.Student" id="getStudentAndTeacherMap">
<id column="sid" property="id"/>
<result column="sname" property="name"/>
<result column="stid" property="tid"/>
<!-- association為關聯物件，在進行多表查詢且相應返回型別的實體類中擁有其他類時使用。property表示資料庫中的表，JavaType表示對應的實體類 -->
<association property="teacher" javaType="cn.sxt.entity.Teacher">
<id column="tid" property="id"/>
<result column="tname" property="name"/>
</association>
</resultMap>
</mapper>5：建立Dao（StudentDao.java）
public class StudentDao {
public List<Student> getStudentAndTeacher() throws IOException{
SqlSession sqlsession = MybatisUtil.getSqlSession();
List<Student> list = sqlsession.selectList("cn.sxt.entity.student-mapper.getStudentAndTeacher");
sqlsession.commit();
return list;
}
}測試類
public class Test {
public static void main(String[] args) throws IOException {
StudentDao dao = new StudentDao();
List<Student> list = dao.getStudentAndTeacher();
for(Student u:list){
System.out.println(u.getId()+" "+u.getName()+" "+u.getTid()+" "+u.getTeacher().getId()+" "+u.getTeacher().getName());
}
}
}測試結果
1 張一 1 1 張三
2 李二 1 1 張三
3 王三 1 1 張三查詢巢狀
1：mapper對映檔案中association標籤的另一種使用方法
<?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="cn.sxt.entity.student-mapper">
<select id="getStudentAndTeacher" resultMap="getStudentAndTeacherMap">
select * from student
</select>
<resultMap type="cn.sxt.entity.Student" id="getStudentAndTeacherMap">
<association property="teacher" column="tid" javaType="cn.sxt.entity.Teacher" select="cn.sxt.entity.teacher-mapper.getTeacher">
</association>
</resultMap>
</mapper>- association標籤中property屬性表示資料庫中的表，column屬性表示外來鍵，JavaType屬性表示實體類，select屬性表示對應表的查詢語句
- select屬性表示對應表的查詢語句，用“包名+檔案名錶示”，一般對於另一表的查詢會寫在另一個mapper對映檔案之內。也可以寫在同一檔案內，若寫在同一檔案內，則包名可省略
- 結果巢狀對資料庫查詢一次，查詢巢狀對資料庫查詢兩次，所以結果巢狀的效率會更高，但查詢巢狀看起來會更清爽點
<?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="cn.sxt.entity.teacher-mapper">
<select id="getTeacher" resultType="cn.sxt.entity.Teacher">
select * from teacher
</select>
</mapper>測試類
public class Test {
public static void main(String[] args) throws IOException {
StudentDao dao = new StudentDao();
List<Student> list = dao.getStudentAndTeacher();
for(Student u:list){
System.out.println(+u.getId()+" "+u.getName()+" "+u.getTid()+" "+u.getTeacher().getId()+" "+u.getTeacher().getName());
}
}測試結果
1 張一 0 1 張三
2 李二 0 1 張三
3 王三 0 1 張三- 此處Student的tid無法查詢，原因未知
一對多
結果巢狀
1：建立mybatis配置檔案（mybatis-config.xml）
2：建立工具類（MybatisUtils.java）用於獲取SqlSession物件
3：建立實體類物件
package cn.sxt.entity;
import java.util.List;
public class Teacher {
private int id;
private String name;
private List<Student> student;
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 List<Student> getStudent() {
return student;
}
public void setStudent(List<Student> student) {
this.student = student;
}
}
- 因為是從Teacher入手的一對多查詢，所以Teacher實體類中需要有list集合的Student屬性
package cn.sxt.entity;
public class Student {
private int id;
private String name;
private int tid;
/*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 int getTid() {
return tid;
}
public void setTid(int tid) {
this.tid = tid;
}
/* public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}*/
}
4：建立mapper對映檔案（teacher-mapper.xml）用於書寫SQL語句
<?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="cn.sxt.entity.teacher-mapper">
<select id="getTeacher" resultType="cn.sxt.entity.Teacher">
select * from teacher
</select>
<select id="getTeacherAndStudent" resultMap="getTeacherAndStudentMap">
select s.id sid,s.name sname,s.tid stid,t.id tid,t.name tname from student s,teacher t where s.tid=t.id and t.id=1
</select>
<resultMap type="cn.sxt.entity.Teacher" id="getTeacherAndStudentMap">
<id column="tid" property="id"/>
<result column="tname" property="name"/>
<collection property="student" ofType="cn.sxt.entity.Student">
<id column="sid" property="id"/>
<result column="sname" property="name"/>
<result column="stid" property="tid"/>
</collection>
</resultMap>
</mapper>- select s.id sid,s.name sname,s.tid stid,t.id tid,t.name tname from student s,teacher t where s.tid=t.id and t.id=1
- 由於Teacher實體類中使用的是關於Student的集合，所以在對映檔案中的resultMap中需要使用collection標籤而不是association標籤
- collection標籤中，property屬性表示資料庫中的表，ofType屬性表示實體類
- 若資料庫中兩個表的列名是相同的，則在查詢時，SQL語句最好要使用別名，便於在resultMap標籤中標明清楚（如上），否則容易出現問題（原因未明）
5：Dao（TeacherDao.java）
public class TeacherDao {
public Teacher getStudentAndTeacher(int id) throws IOException{
SqlSession sqlsession = MybatisUtil.getSqlSession();
Teacher teacher = sqlsession.selectOne("cn.sxt.entity.teacher-mapper.getTeacherAndStudent", id);
sqlsession.commit();
return teacher;
}測試類
public class Test {
public static void main(String[] args) throws IOException {
TeacherDao dao = new TeacherDao();
Teacher teacher = dao.getStudentAndTeacher(1);
System.out.println(teacher.getId()+" "+teacher.getName());
List<Student> list = teacher.getStudent();
for(Student s:list){
System.out.println(s.getId()+" "+s.getName()+" "+s.getTid());
}
}測試結果
1 張三
1 張一 1
2 李二 1
3 王三 1查詢巢狀
1：mapper對映檔案（teacher-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="cn.sxt.entity.teacher-mapper">
<select id="getTeacherAndStudent2" resultMap="getTeacherAndStudent2Map">
select * from teacher where id = #{id}
</select>
<resultMap type="cn.sxt.entity.Teacher" id="getTeacherAndStudent2Map">
<collection property="student" javaType="ArrayList" ofType="cn.sxt.entity.Student" column="id" select="cn.sxt.entity.student-mapper.getStudent">
</collection>
</resultMap>
</mapper>2：Dao
public class TeacherDao {
public Teacher getStudentAndTeacher(int id) throws IOException{
SqlSession sqlsession = MybatisUtil.getSqlSession();
Teacher teacher = sqlsession.selectOne("cn.sxt.entity.teacher-mapper.getTeacherAndStudent2", id);
sqlsession.commit();
return teacher;
}
}測試類
public class Test {
public static void main(String[] args) throws IOException {
TeacherDao dao = new TeacherDao();
Teacher teacher = dao.getStudentAndTeacher(1);
System.out.println(teacher.getId()+" "+teacher.getName());
List<Student> list = teacher.getStudent();
for(Student s:list){
System.out.println(s.getId()+" "+s.getName()+" "+s.getTid());
}
}測試結果
0 張三
1 張一 1
2 李二 1
3 王三 1- 由於在mapper對映檔案的resultMap標籤的collection標籤中使用了column="id"，導致測試結果中Teacher的id無法獲取（原因未知）