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复杂的sql查询

环境搭建

CREATE TABLE `teacher` (
  `id` INT(10) NOT NULL,
  `name` VARCHAR(30) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8

INSERT INTO teacher(id, name) VALUES (1,'秦老师'); 

CREATE TABLE `student` (
  `id` INT(10) NOT NULL,
  `name` VARCHAR(30) DEFAULT NULL,
  `tid` INT(10) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `fktid` (`tid`),
  CONSTRAINT `fktid` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8;
INSERT INTO `student` (`id`, `name`, `tid`) VALUES (1, '小明', 1); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES (2, '小红', 1); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES (3, '小张', 1); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES (4, '小李', 1); 
INSERT INTO `student` (`id`, `name`, `tid`) VALUES (5, '小王', 1);

测试环境搭建

新建实体类Teacher Student
建立Mapper接口
简历Mapper.xml
核心配置文件中绑定注册Mapper接口
测试查询是否能够成功

package com.kuang.pojo;

public class Student { 
   
    private int id;
    private String name;
    //学生需要关联一个老师！
    private Teacher teacher;
}
package com.kuang.pojo;

public class Teacher { 
   
    private int id;
    private String name;
    //一个老师有多个学生
    private List<Student> students;
}

多对一实例

public interface StudentMapper { 
   
    //查询所有的学生信息，以及对应的老师的信息
    public List<Student> getStudent();
}

按照查询嵌套处理

<mapper namespace="com.kuang.dao.StudentMapper">
    <!-- 1.查询所有的学生信息 2.根据查询出来的学生的id,寻找对应的老师 -->
    <select id="getStudent" resultMap="StudentTeacher">
        select  s.id,s.name,t.name from student s,teacher t where s.tid = t.id;
    </select>
    <resultMap id="StudentTeacher" type="Student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <!--复杂的属性，我们需要单独处理 对象:association 集合:collection -->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
    </resultMap>
    <select id="getTeacher" resultType="Teacher">
        select * from mybatis.teacher where id = #{id}
    </select>
    
</mapper>

在这里插入图片描述

按照结果嵌套处理

<!--方式二:-->
    <select id="getStudent" resultMap="StudentTeacher">
        select  s.id sid,s.name sname,t.name tname
        from student s,teacher t
        where s.tid = t.id;
    </select>
    <resultMap id="StudentTeacher" type="Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" column="tname" javaType="Teacher">
            <result property="name" column="tname"/>
        </association>
    </resultMap>

一对多

按照结果嵌套查询


public interface TeacherMapper { 
   
    //获取老师
    List<Teacher> getTeacher();
}

<mapper namespace="com.kuang.dao.TeacherMapper">
    <!--按照查询嵌套处理-->
    <select id="getTeacher" resultMap="TeacherStudent">
        select s.id sid,s.name sname,t.name tname,t.id tid
        from mybatis.student s,mybatis.teacher t
        where s.id = t.id and t.id = #{tid}
    </select>
    <resultMap id="TeacherStudent" type="Teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <collection property="students" ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>
</mapper>

按照查询嵌套处理

<mapper namespace="com.kuang.dao.TeacherMapper">


    <select id="getTeacher" resultMap="TeacherStudent">
        select * from mybatis.teacher where id = #{tid}
    </select>
    <resultMap id="TeacherStudent" type="Teacher">
        <collection property="students" javaType="ArrayList" ofType="Student" select="getStudentByTeacher" column="id"/>
    </resultMap>
    <select id="getStudentByTeacherId" resultType="Student">
        select * from mybatis.student where tid = #{tid}
    </select>
</mapper>

小结

关联-association[多对一]
集合-collection[-对多]
javaType & ofType
- javaType 用来指定
- ofType 用来指定映射到List或者集合中的pojo类型，泛型中的类型

动态SQL

动态SQL就是根据不同的条件生成不同的SQL语句

<select id="findActiveBlogLike" resultType="Blog">
  SELECT * FROM BLOG WHERE state = ‘ACTIVE’
  <if test="title != null">
    AND title like #{title}
  </if>
  <if test="author != null and author.name != null">
    AND author_name like #{author.name}
  </if>
</select>