MyBatisPlus中条件查询怎么用Wrapper来构建?

增删改查四个操作中，查询是非常重要的也是非常复杂的操作，MyBatisPlus将书写复杂的SQL查询条件进行了封装，使用编程的形式完成查询条件的组合。 在查询所有和分页查询的时候，都能看到过一个Wrapper类，这个类就是用来构建查询条件的，如下图所示:

环境构建

在构建条件查询之前，我们先来准备下环境： 创建一个SpringBoot项目和pom.xml中添加对应的依赖。

<?xml version="1.0" encoding="UTF-8"?>
<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 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.5.0</version>
    </parent>
    <groupId>com.itheima</groupId>
    <artifactId>mybatisplus_02_dql</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <properties>
        <java.version>1.8</java.version>
    </properties>
        <dependencies>

        <dependency>
            <groupId>com.baomidou</groupId>
            <artifactId>mybatis-plus-boot-starter</artifactId>
            <version>3.4.1</version>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>

        <dependency>
            <groupId>com.alibaba</groupId>
            <artifactId>druid</artifactId>
            <version>1.1.16</version>
        </dependency>

        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <scope>runtime</scope>
        </dependency>

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>

        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
        </dependency>

        </dependencies>

        <build>
            <plugins>
                <plugin>
                     <groupId>org.springframework.boot</groupId>
                     <artifactId>spring-boot-maven-plugin</artifactID>
                 </plugin>
              </plugins>
          </build>
      </project>

编写UserDao接口

@Mapper   
public  interface UserDao  extends  BaseMapper<User>  {   
}

编写模型类

@Data
public class User {
    private Long id;
    private String name;
    private String password;
    private Integer age;
    private String tel;
 }

编写引导类

@SpringBootApplication
public class Mybatisplus02DqlApplication {

    public static void main(String[] args) {
        SpringApplication.run(Mybatisplus02DqlApplication.class, args);
    }

}

编写配置文件

# dataSource
spring:
  datasource:
    type: com.alibaba.druid.pool.DruidDataSource
    driver-class-name: com.mysql.cj.jdbc.Driver
    url: jdbc:mysql://localhost:3306/mybatisplus_db?serverTimezone=UTC
    username: root
    password: root
# mp日志
mybatis-plus:
  configuration:
    log-impl: org.apache.ibatis.logging.stdout.StdOutImpl

编写测试类

@SpringBootTest
class Mybatisplus02DqlApplicationTests {

    @Autowired
    private UserDao userDao;

    @Test
    void testGetAll(){
         List<User> userList = userDao.selectList(null);
         System.out.println(userList);
     }
 }

最终创建的项目结构为:

测试的时候，控制台打印的日志比较多，速度有点慢而且不利于查看运行结果，所以接下来我们把 这个日志处理下:

取消初始化spring日志打印，resources目录下添加logback.xml，名称固定，内容如下:

<?xml version="1.0" encoding="UTF-8"?> 
<configuration> 
</configuration>

application.yml添加如下内容

# mybatis-plus日志控制台输出
mybatis-plus:
  configuration:
    log-impl: org.apache.ibatis.logging.stdout.StdOutImpl
  global-config:
    banner: off # 关闭mybatisplus启动图标

取消SpringBoot的log打印

application.yml添加如下内容:

spring:
  main:
    banner-mode: off # 关闭SpringBoot启动图标(banner)