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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

1. 新建实体类Teacher Student
2. 建立Mapper接口
3. 简历Mapper.xml
4. 核心配置文件中绑定注册Mapper接口
5. 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>

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

1. 关联-association[多对一]
2. 集合-collection[-对多]
3. 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>

foreach