mybatis（三）MyBatis的关联查询以及$和#的区别

联表查询

-- 创建表和数据：
create database mybatis;
use mybatis;
CREATE TABLE users(id INT PRIMARY KEY AUTO_INCREMENT, NAME
VARCHAR(20), age INT);
INSERT INTO users(NAME, age) VALUES('Tom', 12);
INSERT INTO users(NAME, age) VALUES('Jack', 11);

CREATE TABLE student(
s_id INT PRIMARY KEY AUTO_INCREMENT,
s_name VARCHAR(20),
class_id INT
);

需要被联表的类需要在主类中关联

例如老师学生，对于老师是一对多，而对于学生则是多对一，一名老师多个学生

//班级
public class Clazz {
    private int cId;
    private String cName;
    private String teacherId;
    private Teacher teacher;    //将Teacher封装到班级里
    //get,set      
}
//老师
public class Teacher {
    private int id;
    private String name;
    //get,set   
}

一对一：（sql语句的位置无关紧要，可以在下面也可以在上面）

方式一：嵌套结果：使用嵌套结果映射来处理重复的联合结果的子集

封装联表查询的数据(去除重复的数据)

<select id="getUsers" resultMap="ClazzMapper">
        select * from class c,teacher t where t.t_id=c.teacher_id and c.c_id=#{cid}
    </select>
    
    <resultMap type="com.zhiyou100.wyf.bean.Clazz" id="ClazzMapper">
        <id column="c_id" property="cid"/>
        <result column="c_name" property="cname"/>
        <association property="teacher" column="teacher_id" javaType="com.zhiyou100.wyf.bean.Teacher">
            <id property="tid" column="t_id"/>
            <result property="tname" column="t_name"/>
        </association>
    </resultMap>

方式二：嵌套查询：通过执行另外一个 SQL 映射语句来返回预期的复杂类型

SELECT * FROM class WHERE c_id=1;

SELECT * FROM teacher WHERE t_id=1 //1 是上一个查询得到的 teacher_id 的值

<resultMap type="com.zhiyou100.wyf.bean.Clazz" id="ClazzMapper">
        <id column="c_id" property="cid"/>
        <result column="c_name" property="cname"/>
        <result column="teacher_id" property="teacher_id"/>
        <association property="teacher" column="teacher_id"  select="com.zhiyou100.wyf.dao.TeacherDao.getTeacher" javaType="com.zhiyou100.wyf.bean.Teacher">
        </association>
    </resultMap>
    <select id="getUsers" resultMap="ClazzMapper">
        select * from class where c_id =#{cid}
    </select>

 <select id="getTeacher" resultType="com.zhiyou100.wyf.bean.Teacher">
        select t_id tid,t_name tname  from teacher where t_id=#{tid}
    </select>

 

一对多：

创建表和数据

CREATE TABLE student(
s_id INT PRIMARY KEY AUTO_INCREMENT,
s_name VARCHAR(20),
class_id INT
);
INSERT INTO student(s_name, class_id) VALUES('xs_A', 1);
INSERT INTO student(s_name, class_id) VALUES('xs_B', 1);
INSERT INTO student(s_name, class_id) VALUES('xs_C', 1);
INSERT INTO student(s_name, class_id) VALUES('xs_D', 2);
INSERT INTO student(s_name, class_id) VALUES('xs_E', 2);
INSERT INTO student(s_name, class_id) VALUES('xs_F', 2);

方式一: 嵌套结果: 使用嵌套结果映射来处理重复的联合结果的子集

SELECT * FROM class c, teacher t,student s WHERE c.teacher_id=t.t_id AND c.C_id=s.class_id AND c.c_id=1

<resultMap type="com.zhiyou100.wyf.bean.Clazz" id="ClazzMapper">
        <id column="c_id" property="cid"/>
        <result column="c_name" property="cname"/>
        <result column="teacher_id" property="teacher_id"/>
        <association property="teacher" column="teacher_id" 
            select="com.zhiyou100.wyf.dao.TeacherDao.getTeacher" javaType="com.zhiyou100.wyf.bean.Teacher">
        </association>
        <collection property="student" ofType="com.zhiyou100.wyf.bean.Student">
            <id property="s_id" column="s_id"/>
            <result property="s_name" column="s_name"/>
        </collection>
    </resultMap>
    <select id="getUsers" resultMap="ClazzMapper">
        select * from class c,student s,teacher t 
        where c.teacher_id=t.t_id and s.class_id=c.c_id and c.c_id=#{cid}
    </select>

方式二：嵌套查询：通过执行另外一个 SQL 映射语句来返回预期的复杂类型

SELECT * FROM class WHERE c_id=1;

SELECT * FROM teacher WHERE t_id=1 //1 是上一个查询得到的 teacher_id 的值

SELECT * FROM student WHERE class_id=1 //1 是第一个查询得到的 c_id 字段的值

<mapper namespace="com.zhiyou100.wyf.dao.ClazzDao">
    <select id="selectById" parameterType="int" resultMap="mymap">
        select * from class where c_id=#{cid}
    </select>
    <select id="selectByTeacherId" resultType="com.zhiyou100.wyf.bean.Teacher">
        select t_id tid,t_name tname from teacher where t_id=#{tid}
    </select>
    <select id="selectByClassId" resultType="com.zhiyou100.wyf.bean.Student">
        select s_id id,s_name name from student where class_id=#{classid}
    </select>
    <resultMap type="com.zhiyou100.wyf.bean.Clazz" id="mymap">
        <id column="c_id" property="cId"/>
        <result column="c_name" property="cName"/>
        <result column="teacher_id" property="teacherId"/>
        <association property="teacher" javaType="com.zhiyou100.wyf.bean.Teacher" 
        column="teacher_id" select="selectByTeacherId"></association>
        <collection property="students" ofType="com.zhiyou100.wyf.bean.Student" select="selectByClassId" column="c_id"></collection>
    </resultMap>
</mapper>

注意：Mybatis中$和#的区别

1.#{}是预编译处理，${}是字符串替换。

2.Mybatis在处理#{}时，会将sql中的#{}替换为?号，调用PreparedStatement的set方法来赋值；

3.Mybatis在处理${}时，就是把${}替换成变量的值。

4.使用#{}可以有效的防止SQL注入，提高系统安全性。

https://www.cnblogs.com/xdp-gacl/p/4262895.html