MyBatis联表处理

多对一的处理

• 数据库结构
  
  
  

• 实体类

public class Student {
    private int id;
    private String name;
    //多个学生对应一个老师
    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;
    }
}

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;
    }
}

• 编写student.mapper.xml映射文件

  • 结果嵌套处理

    <!-- 按结果嵌套处理 -->
    <select id="getStudents" resultMap="StudentTeacher">
        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="Student" id="StudentTeacher">
        <id column="sid" property="id"/>
        <result column="sname" property="name"/>
        <!-- 关联对象 -->
        <!-- property关联对象在Student实体类中的属性 -->
        <association property="teacher" javaType="Teacher">
            <id column="tid" property="id"/>
            <result column="tname" property="name"/>
        </association>
    </resultMap>

    • resultMap中通过<association>嵌套了teacher属性

  • 查询嵌套处理

    <!-- 按查询嵌套处理 -->
    <select id="getStudents2" resultMap="StudentTeacher2">
        select * from student
    </select>
    <resultMap type="Student" id="StudentTeacher2">
        <!-- association关联属性 -->
        <!-- property属性名 -->
        <!-- column关联属性在多的一方的列名 -->
        <!-- javaType关联属性的类型 -->
        <!-- select关联查询 -->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher">
        </association>
    </resultMap>
    <select id="getTeacher" resultType="Teacher">
        select * from teacher where id=#{id}
    </select>

---

一对多的处理

• 数据库结构
  
  
  

• 实体类

public class Teacher {
    private int id;
    private String name;
    //一个老师对应多个学生
    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 List<Student> getStudents() {
        return students;
    }
    public void setStudents(List<Student> students) {
        this.students = students;
    }
}

public class Student {
    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;
    }
}

• 编写teacher.mapper.xml

  • 按结果嵌套处理

    <select id="getTeacher" resultMap="TeacherStudent">
    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=#{id}
    </select>
    <resultMap type="Teacher" id="TeacherStudent">
    <id column="tid" property="id"/>
    <result column="tname" property="name"/>
    <!-- association是关联一个 -->
    <!-- collection是关联多个 -->
    <collection property="students" ofType="Student">
        <id column="sid" property="id"/>
        <result column="sname" property="name"/>
    </collection>
    </resultMap>

    • <resultMap使用的标签是<collection>
    • <collection>中属性是ofType

  • 按查询嵌套处理

    <select id="getTeacher2" resultMap="TeacherStudent2">
        select * from teacher where id=#{id}
    </select>
    <resultMap type="Teacher" id="TeacherStudent2">
        <!-- column是一端的主键列名 -->
        <collection property="students" javaType="ArrayList" ofType="Student" column="id" select="getStudentByTid"></collection>
    </resultMap>
    <select id="getStudentByTid" resultType="Student">
         select * from student where tid=#{id}
    </select>

---

动态sql

• Dynamic sql
  
  • 动态sql: 根据不同饿查询条件，生成不同的查询语句
  • 通过<select>中的<where>和<if>标签实现判断

<mapper namespace="com.eric.entity.UserMapper">
    <select id="getUserByCondition" parameterType="Map" resultType="User">
        select * from user
        <where>
            <if test="name!=null">
                name like CONCAT('%', #{name}, '%')
            </if>
        </where>
    </select>
</mapper>