Association mybatis (three) MyBatis query and the difference between the $ and #

United-table query

-- 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
);

You need to be associated with the class table in the main category

For example student teacher, for teachers-many, and many-is for students, a teacher multiple student

// Class 
public class Clazz { 
    Private int CID; 
    Private String cName; 
    Private String teacherId; 
    Private Teacher Teacher; // the class encapsulates the Teacher 
    // GET, SET       
} 
// teacher 
public class Teacher { 
    Private int ID; 
    Private String name; 
    // GET, SET    
}

One to One: (position does not matter sql statement, may also be below the above)

One way: Nested Results: By using nested result mappings combined result of the processing to repeat the subset

Package data table associated query (remove duplicate data)

<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>

 

Second way: nested query: returns expected by executing another mapped SQL statement is complex type

SELECT * FROM class WHERE c_id=1;

Value SELECT * FROM teacher WHERE t_id = 1 // 1 is on a query of the resulting teacher_id

<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>

 

Many:

Creating tables and data

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);

 

A mode : Nested Results : By using nested result mapping result of the joint processing to duplicate a subset of

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>

Second way: Nesting: By executing another SQL to return the expected complex type mapped statement

SELECT * FROM class WHERE c_id=1;

SELECT * FROM teacher WHERE t_id = 1 // 1 is a query to get the teacher_id value

SELECT * FROM student WHERE class_id = 1 // 1 is obtained by the first query c_id value field

<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>

 

 

 

Note: Mybatis the difference between the $ and #

1. # {} Is a pre-compiler process, $ {} string is replaced.

2.Mybatis when processing # {}, will be in sql number # {} with the call set assignment method PreparedStatement to?;

When processing 3.Mybatis $ {}, {} $ is to replace the value of the variable.

4.} # {SQL injection can effectively prevent, improve system security.

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