MyBatis 动态SQL之＜set＞标签

简介

在 Mybatis 中，update 语句可以使用 set 标签动态更新列。set 标签可以为 SQL 语句动态的添加 set 关键字，剔除追加到条件末尾多余的逗号。
作用:
自动在要修改的第一个字段之前添加SET关键字
去掉要修改的第一个字段前的连接符(,)

语法

set>
set>

网络案例

    
    update id="updateWebsite"
        parameterType="net.biancheng.po.Website">
        UPDATE website
        set>
            if test="name!=null">name=#{name}，if>
            if test="url!=null">url=#{url}，if>
        set>
        WHERE id=#{id}
    update>

基础环境

1.数据库准备

# 创建一个名称为t_customer的表
CREATE TABLE t_customer (
    id int(32) PRIMARY KEY AUTO_INCREMENT,
    username varchar(50),
    jobs varchar(50),
    phone varchar(16)
);
# 插入3条数据
INSERT INTO t_customer VALUES ('1', 'joy', 'teacher', '13733333333');
INSERT INTO t_customer VALUES ('2', 'jack', 'teacher', '13522222222');
INSERT INTO t_customer VALUES ('3', 'tom', 'worker', '15111111111');

2.新建项目或Module

3 pom.xml中添加

project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    parent>
        artifactId>mybatisartifactId>
        groupId>com.examplegroupId>
        version>1.0-SNAPSHOTversion>
    parent>
    modelVersion>4.0.0modelVersion>

    groupId>com.biemgroupId>
    artifactId>dynamaicSqlartifactId>

    properties>
        maven.compiler.source>8maven.compiler.source>
        maven.compiler.target>8maven.compiler.target>
    properties>

    dependencies>
        
        dependency>
            groupId>org.mybatisgroupId>
            artifactId>mybatisartifactId>
            version>3.4.6version>
        dependency>
        
        dependency>
            groupId>junitgroupId>
            artifactId>junitartifactId>
            version>4.12version>
            scope>testscope>
        dependency>

        
        dependency>
            groupId>mysqlgroupId>
            artifactId>mysql-connector-javaartifactId>
            version>8.0.18version>
            scope>runtimescope>
        dependency>

        
        dependency>
            groupId>log4jgroupId>
            artifactId>log4jartifactId>
            version>1.2.17version>
        dependency>

        dependency>
            groupId>org.projectlombokgroupId>
            artifactId>lombokartifactId>
            version>1.18.16version>
        dependency>
    dependencies>
project>

4.创建package和文件夹

src/main/java/下创建package
com.biem.pojo
com.biem.mapper
com.biem.util
src/main/resources/下创建文件夹
com/biem/mapper
src/test/java下创建package
com.biem.test

5 框架配置文件

5.1 mybatis核心配置文件mybatis-config.xml

DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
configuration>
    
    properties resource="jdbc.properties">properties>

    
    settings>
        setting name="mapUnderscoreToCamelCase" value="true"/>
    settings>

    
    typeAliases>
        
        package name="com.biem.pojo"/>
    typeAliases>

    environments default="development">
        environment id="development">
            transactionManager type="JDBC"/>
            dataSource type="POOLED">
                property name="driver" value="${jdbc.driver}"/>
                property name="url" value="${jdbc.url}"/>
                property name="username" value="${jdbc.username}"/>
                property name="password" value="${jdbc.password}"/>
            dataSource>
        environment>

    environments>

    
    mappers>
        
        package name="com.biem.mapper"/>
    mappers>


configuration>

5.2 mybatis属性文件jdbc.properties

jdbc.driver=com.mysql.cj.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/mybatis?serverTimezone=UTC
jdbc.username=root
jdbc.password=root

5.3 log4j.xml文件

DOCTYPE log4j:configuration SYSTEM "log4j.dtd">

log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/">

    appender name="STDOUT" class="org.apache.log4j.ConsoleAppender">
        param name="Encoding" value="UTF-8"/>
        layout class="org.apache.log4j.PatternLayout">
            param name="ConversionPattern" value="%-5p %d{MM-dd HH:mm:ss,SSS} %m (%F:%L) n"/>
        layout>
    appender>
    logger name="java.sql">
        level value="debug"/>
    logger>
    logger name="org.apache.ibatis">
        level value="info"/>
    logger>
    root>
        level value="debug"/>
        appender-ref ref="STDOUT"/>
    root>
log4j:configuration>

6 用户配置文件

6.1 实体类

package com.biem.pojo;

import lombok.*;

/**
 * ClassName: Customer
 * Package: com.biem.pojo
 * Description:
 *
 * @Create 2023/4/5 22:17
 * @Version 1.0
 */
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
@Builder
@ToString
public class Customer {
    private Integer id;
    private String username;
    private String jobs;
    private String phone;
}

需要在pom.xml中引入lombok，简化原来的实体类的代码

6.2 mybatis接口类

package com.biem.mapper;

/**
 * ClassName: CustomerMapper
 * Package: com.biem.mapper
 * Description:
 *
 * @Create 2023/4/5 22:19
 * @Version 1.0
 */
public interface CustomerMapper {
}

6.3 mybatis用户配置文件

DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
mapper namespace="com.biem.mapper.CustomerMapper">
    

    

mapper>

6.4 mybatis工具类

package com.biem.util;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.IOException;
import java.io.InputStream;

/**
 * ClassName: MybatisUtil
 * Package: com.biem.utils
 * Description:
 *
 * @Create 2023/4/5 22:23
 * @Version 1.0
 */
public class MybatisUtil {
    //利用static(静态)属于类不属于对象,且全局唯一
    private static SqlSessionFactory sqlSessionFactory = null;
    //利用静态块在初始化类时实例化sqlSessionFactory
    static {
        InputStream is= null;
        try {
            is = Resources.getResourceAsStream("mybatis-config.xml");
            sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);
        } catch (IOException e) {
            e.printStackTrace();
            throw new ExceptionInInitializerError(e);
        }
    }

    /**
     * openSession 创建一个新的SqlSession对象
     * @return SqlSession对象
     */
    public static SqlSession openSession(boolean autoCommit){
        return sqlSessionFactory.openSession(autoCommit);
    }

    public static SqlSession openSession(){
        return sqlSessionFactory.openSession();
    }
    /**
     * 释放一个有效的SqlSession对象
     * @param session 准备释放SqlSession对象
     */
    public static void closeSession(SqlSession session){
        if(session != null){
            session.close();
        }
    }
}

6.5 测试类

package com.biem.test;

import com.biem.mapper.CustomerMapper;
import com.biem.pojo.Customer;
import com.biem.util.MybatisUtil;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import java.util.List;

/**
 * ClassName: TestCustomer
 * Package: com.biem.test
 * Description:
 *
 * @Create 2023/4/5 22:32
 * @Version 1.0
 */
public class TestCustomer {
}

7 项目结构如下

使用set标签更新记录

1 com.biem.mapper.CustomerMapper.class中添加

public int updateCustomerBySet(Customer customer);

2 com/biem/mapper/CustomerMapper.xml中添加

    
    update id="updateCustomerBySet" parameterType="customer">
        update t_customer
        set>
            if test="username != null and username != ''">
                username=#{username}
            if>
            if test="jobs != null and jobs != ''">
                jobs=#{jobs}
            if>
            if test="phone != null and phone != ''">
                phone=#{phone}
            if>
        set>
        where id = #{id}
    update>

3.功能测试 com.biem.test.TestMybatis.java添加测试方法

    @Test
    public void testUpdateCustomerBySet(){
        // 通过工具类获取SqlSession对象
        SqlSession session = MybatisUtil.openSession();
        // 创建Customer对象，封装需要组合查询的条件
        Customer customer = new Customer();
        customer.setId(3);
        customer.setPhone("15111001100");
        CustomerMapper mapper = session.getMapper(CustomerMapper.class);
        int rows = mapper.updateCustomerBySet(customer);
        if(rows>0){
            System.out.println("您成功的更新了"+rows+"条记录");
        }else{
            System.out.println("更新失败");
        }
        // 关闭SqlSession
        session.close();
    }