Mybatis learning two (field names and attribute names are not the same entity class / relational query)

1. At the time of writing the project will encounter a database field names and the name of the entity class attribute different circumstances, might be for the convenience may also be other requirements, whatever, we are to solve this problem

Prepare a database table is as follows

1 CREATE TABLE orders(
2 　　order_id INT PRIMARY KEY AUTO_INCREMENT,
3 　　order_no VARCHAR(20),
4 　　order_price FLOAT
5 ）;

We took the class definition of an entity class

1 public class Order {
2     private int id;
3     private String orderNo;
4     private float price;
5 }

Visible entity class and database table field names we define inconsistent, there are two solutions for this

Method One: Define alias sql statement

1 <select id="selectOrder" parameterType="int" resultType="_Order">
2     select order_id id, order_no orderNo,order_price price from orders where order_id=#{id}
3 </select>

Method two: solved by <resultMap>

<select id="selectOrderResultMap" parameterType="int" resultMap="orderResultMap">
     select * from orders where order_id=#{id}
</select>
<resultMap type="_Order" id="orderResultMap">
    <id property="id" column="order_id"/>
    <result property="orderNo" column="order_no"/>
    <result property="price" column="order_price"/>
</resultMap>

For example (which relates to the nested ( Association ) associated with the query results, the following will explain it)

2. Implement contingency table query

Defined entity classes

teacher entity class

private int tId;
private String tName;

student entity class

private int sId;
private String sName;
private int ClassId;

clazz entity class

private int cId;
private String cName;
private int tId;
private Teacher teacher;
private List<Student> students;

Wherein clazz entity class, a class corresponding to a teacher, a class corresponding to a plurality of students

At this time, the following mapping file ClazzMapper.xml

For many, many to one question we can have two solutions

Method One: Secondary Query

Method Two: Union-table query

. 1  <? XML Version = "1.0" encoding = "UTF-. 8" ?> 
2  <! DOCTYPE Mapper the PUBLIC "- // mybatis.org//DTD Mapper 3.0 // EN"
 . 3  "http://mybatis.org/ DTD /. 3-MyBatis-mapper.dtd " > 
. 4  <-! namespace: namespace represented. Now the aim is to distinguish id. -> 
. 5  < Mapper namespace = "com.zhiyou100.zhl.dao.ClazzDao" > 
. 6      < The resultMap type = "com.zhiyou100.zhl.bean.Clazz" id = "myMap" > 
. 7          < ID column = "c_id" Property = "
         column="c_name" property="cName"/>
 9         <result column="teacher_id" property="tId"/>
10         <association property="teacher" javaType="com.zhiyou100.zhl.bean.Teacher">
11             <id column="t_id" property="tId"/>
12             <result column="t_name" property="tName"/>
13         </association>
14         <collection property="students" ofType="com.zhiyou100.zhl.bean.Student">
15             <id column="s_id" property="sId"/>
16             <result column="s_name" property="sName"/>
17         </collection>
18     </resultMap>
19     
20     <select id="selectById" resultMap="mymap">
21         select * from class c join teacher t join student s on c.teacher_id=t.t_id and s.class_id=c.c_id where c_id=#{cId};
22     </select>
23     
24 </mapper>

Unit testing

Mybatis learning two (field names and attribute names are not the same entity class / relational query)

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