【Video & Communication Platform】
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https://gitee.com/happyangellxq520/spring-boot
à Spring Boot communication platform
http://412887952-qq-com.iteye.com/blog/2321532
Origin of need :
In the previous article, we introduced how to perform insert operations. In practice, we also have query, modification, and deletion operations, which are often used in projects. Let's take a look at the outline of this section:
Outline of this section :
(2) delete: query operation
(3) select all: query all
(4) select by id: query operation according to id
(5) update off-topic
(6) code
Next, look at the specific content:
( 1 ) update : modification operation
Code on the basis of the original code, and add in Demo.xml :
<update id="update">
update demo set name = #{name} where id = #{id}
</update>
Add the following code to DemoMapper :
public int update(@Param("id")int id,@Param("name")String name);
Note: The int value is returned here . This is the number of successful modifications returned by the value. For example, if 5 pieces of data are successfully found and the modification is successful, then the return value is 5 .
( 2 ) delete : query operation
Add to the Demo.xml file:
<delete id="delete">
delete from demo where id = #{id}
</delete>
在DemoMapper中加入：
public int delete(int id);
注意：返回值代表成功删除了多少条数据。
（3）select all： 查询全部
在Demo.xml中加入：
<resultMap id="baseResultMap" type="com.kfit.demo.bean.Demo" >
<id property="id" column="id" />
<result property="name" column="name" />
</resultMap
<select id="selectAll" resultMap="baseResultMap">
select *from demo
</select>
在DemoMapper类中加入：
public List<Demo> selectAll();
使用@Select注明这是select语句。
（4）select by id：根据id查询操作
在Demo.xml文件中加入：
<select id="selectById" resultMap="baseResultMap">
select *from demo where id = #{id}
</select>
在DemoMapper类中加入：
public Demo selectById(int id);
（5）update题外话
在DemoMapper中，update是指定了多个参数，但是实际中一个实体类中会有很多的字段，这种方式，总感觉不是很好，那么有更好的方式呢，有的，如下的写法：
原先写法：
public int update(@Param("id")int id,@Param("name")String name);
对象写法：
public int update(Demo demo);
修改完之后瞬间清爽了很多，但是原先的写法也是需要会的，使用场景，比如：我们要修改状态status的话，那么使用updateStatus(int id,int status)的方式，直接指定会更好。
（6）代码
Demo.xml配置文件代码如下：
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.kfit.demo.mapper.DemoMapper">
<resultMap id="baseResultMap" type="com.kfit.demo.bean.Demo" >
<id property="id" column="id" />
<result property="name" column="name" />
</resultMap>
<!-- insert 语句. -->
<insert id="save" parameterType="com.kfit.demo.bean.Demo" useGeneratedKeys="true" keyProperty="id">
insert into demo (name) values (#{name})
</insert>
<update id="update">
update demo set name = #{name} where id = #{id}
</update>
<delete id="delete">
delete from demo where id = #{id}
</delete>
<select id="selectAll" resultMap="baseResultMap">
select *from demo
</select>
<select id="selectById" resultMap="baseResultMap">
select *from demo where id = #{id}
</select>
</mapper>
DemoMapp.java代码如下：
package com.kfit.demo.mapper;
import java.util.List;
import org.apache.ibatis.annotations.Param;
import com.kfit.demo.bean.Demo;
public interface DemoMapper {
public int save(Demo demo);
public int update(@Param("id")int id,@Param("name")String name);
public int delete(int id);
public List<Demo> selectAll();
public Demo selectById(int id);
}
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http://412887952-qq-com.iteye.com/blog/2321532