使用Mybatis如何实现多对一或一对多的关联查询
使用Mybatis 如何实现多对一或一对多的关联查询?针对这个问题,这篇文章详细介绍了相对应的分析和解答,希望可以帮助更多想解决这个问题的小伙伴找到更简单易行的方法。
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首先 数据库量表之间字段关系(没有主外键)
studentmajor表的id字段对应student表里major字段
两个实体类
package com.model; import java.util.Date; public class Student { private Integer sno; private String sname; private String ssex; private Integer sclass; private StudentMajor studentmajor; public Student() { super(); } public Student(Integer sno, String sname, String ssex, Integer sclass, StudentMajor studentmajor) { super(); this.sno = sno; this.sname = sname; this.ssex = ssex; this.sclass = sclass; this.studentmajor = studentmajor; } public StudentMajor getStudentmajor() { return studentmajor; } public void setStudentmajor(StudentMajor studentmajor) { this.studentmajor = studentmajor; } public Integer getSno() { return sno; } public void setSno(Integer sno) { this.sno = sno; } public String getSname() { return sname; } public void setSname(String sname) { this.sname = sname; } public String getSsex() { return ssex; } public void setSsex(String ssex) { this.ssex = ssex; } @Override public String toString() { return "Student [sno=" + sno + ", sname=" + sname + ", ssex=" + ssex + ", sclass=" + sclass + ", studentmajor=" + studentmajor + "]"; } public Integer getSclass() { return sclass; } public void setSclass(Integer sclass) { this.sclass = sclass; } }
package com.model; import java.util.List; public class StudentMajor { private Integer id; private String mcode; private String mname; private Liststudents; public StudentMajor() { super(); } public StudentMajor(Integer id, String mcode, String mname, List students) { super(); this.id = id; this.mcode = mcode; this.mname = mname; this.students = students; } @Override public String toString() { return "StudentMajor [id=" + id + ", mcode=" + mcode + ", mname=" + mname + ", students=" + students + "]"; } public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getMcode() { return mcode; } public void setMcode(String mcode) { this.mcode = mcode; } public String getMname() { return mname; } public void setMname(String mname) { this.mname = mname; } public List getStudents() { return students; } public void setStudents(List students) { this.students = students; } }
定义两个接口
package com.dao; import java.util.List; import java.util.Map; import com.model.Student; public interface StudentMapper { /** * 全表查询 */ public Listselectall(); /** * 根据专业查人员,给一对多用 */ public List selectz(Integer major); }
package com.dao; import java.util.List; import com.model.StudentMajor; public interface StudentMajorMapper { /** * 全表查询 * @return */ public ListselectAll(); /** * 根据主键查数据,给多对一用 * @param id * @return */ public StudentMajor select(Integer id); }
定义两个实体类的映射方法
<?xml version="1.0" encoding="UTF-8"?>
<?xml version="1.0" encoding="UTF-8"?>
JUnit测试
package com.util; import java.util.List; import org.apache.ibatis.session.SqlSession; import org.junit.After; import org.junit.Before; import org.junit.Test; import com.dao.StudentMajorMapper; import com.dao.StudentMapper; import com.model.Student; import com.model.StudentMajor; public class JJJtest { private SqlSession ss; private StudentMapper sm; private StudentMajorMapper smm; @Before public void setUp() throws Exception { ss=SqlSessionUtil.getSqlSession(); sm=ss.getMapper(StudentMapper.class); smm=ss.getMapper(StudentMajorMapper.class); } @After public void tearDown() throws Exception { ss.commit(); ss.close(); } //一对多查询 public void test() { Listlist=smm.selectAll(); for(StudentMajor a:list){ System.out.println(a); } } //根据专业查人员,给一对多用 public void selectz(){ List l=sm.selectz(3); for(Student a:l){ System.out.println(a); } } //多对一查询 @Test public void selectall() { List st=sm.selectall(); for(Student tt:st){ System.out.println(tt); } } //根据主键查询,给多对一用 public void select(){ StudentMajor a=smm.select(1); System.out.println(a); } }
一对多查询结果
多对一查询结果
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