三，MyBatis-Plus 的各种查询的“超详细说明”，比如(等值查询，范围查询，模糊查询...)

三，MyBatis-Plus 的各种查询的“超详细说明”，比如(等值查询，范围查询，模糊查询...)

@

目录

• 三，MyBatis-Plus 的各种查询的“超详细说明”，比如(等值查询，范围查询，模糊查询...)
• 1. 条件构造器介绍
• 2. 准备工作：
• 3. 等值查询
  • 3.1 eq (条件筛选属性 = ?)
  • 3.2 allEq(满足多个条件字段的值的筛选)
  • 3.3 ne (不等于)
• 4. 范围查询
  • 4.1 gt( > 大于的范围)
  • 4.2 ge(>= 大于等于的范围)
  • 4.3 lt(< 小于的范围)
  • 4.4 le(小于等于的范围)
  • 4.5 between(在该区间的范围)
  • 4.6 notBetween(不在该区间的范围)
• 5. 模糊查询
  • 5.1 like( %值%)
  • 5.2 notLike(不满足 %值%)
  • 5.3 likeLeft（%值）
  • 5.4 likeRight(值%)
• 6. 判空查询
  • 6.1 isNUll(判断是否为 Null )
  • 6.2 isNotNull
• 7. 包含查询
  • 7.1 in（包含该内容的字段）
  • 7.2 notIn(不包含该内容的字段)
  • 7.3 inSql(包含该内容的字段)
  • 7.4 notInSql(不包含该内容的字段)
• 8. 分组查询
  • 8.1 groupBy(分组的字段)
• 9. 聚合查询
  • 9.1 having(聚合的字段)
• 10. 排序查询
  • 10.1 orderByAsc(升序)
  • 10.2 orderByDesc(降序)
  • 10.3 orderBy(多字段排序定义)
• 11. func 查询
• 12. 逻辑查询
  • 12.1 and(与)
  • 12.2 or(或)
  • 12.3 nested(非)
• 13. 自定义条件查询
  • 13.1 apply(自定义查询条件)
• 14. last 查询(在sql语句的最末尾拼接“字符串”)
• 15. exists查询
  • 15.1 exists(存在查询)
  • 15.2 notExists(不存在查询)
• 16. 字段查询
  • 16.1 select(字段查询)
• 17. 最后：

---

1. 条件构造器介绍

在实际开发需求中条件查询是非常普遍的，接下来我们就来讲解如何使用 MyBatis Plus 完成条件查询。

首先，想要使用 MyBatis Plus 完成条件查询，基于面向对象的思想，万物皆对象 ，那么查询条件，也需要使用对象来完成封装。我们可以先看看，在 MyBatis Plus 中和条件有关的类有哪些，他们之间有什么关系，理清楚了这个，我们在传递条件对象的时候，就很清晰了。

Wrapper 抽象类，条件类的顶层，提供了一些获取和判断相关的方法。

AbstractWrapper 抽象类，Wrapper 的子类，提供了所有的条件相关方法。

AbstractLambdaWrapper 抽象类，AbstractWrapper 的子类，确定字段参数为方法引用类型。

QueryWrapper 类，AbstractWrapper 的子类，如果我们需要传递 String 类型的字段信息，创建该对象。

LambdaQueryWrapper 类，AbstractLambdaWrapper 的子类，如果我们需要传递方法引用方式的字段信息，创建该对象。

该图为以上各类的关系，我们在编写代码的时候，只需要关注 QueryWrapper 和LambdaQueryWrapper

---

2. 准备工作：

引入相关的jar 依赖。在 pom.xml 文件当中；

<?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>2.5.3</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.rainbowsea</groupId>
    <artifactId>mp03</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>mp03</name>
    <description>mp03</description>
    <url/>
    <licenses>
        <license/>
    </licenses>
    <developers>
        <developer/>
    </developers>
    <scm>
        <connection/>
        <developerConnection/>
        <tag/>
        <url/>
    </scm>
    <properties>
        <java.version>8</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>

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

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

        <!--        mysql 驱动依赖-->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
        </dependency>



        <!--        lombok 的依赖-->
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
        </dependency>

        <!--        druid-->
        <dependency>
            <groupId>com.alibaba</groupId>
            <artifactId>druid</artifactId>
            <version>1.2.8</version>
        </dependency>

        <!--        mybatis-plus 的依赖-->
        <dependency>
            <groupId>com.baomidou</groupId>
            <artifactId>mybatis-plus-boot-starter</artifactId>
            <version>3.4.3</version>
        </dependency>

    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
    <repositories>
        <repository>
            <id>spring-snapshots</id>
            <name>Spring Snapshots</name>
            <url>https://repo.spring.io/snapshot</url>
            <releases>
                <enabled>false</enabled>
            </releases>
        </repository>
    </repositories>
    <pluginRepositories>
        <pluginRepository>
            <id>spring-snapshots</id>
            <name>Spring Snapshots</name>
            <url>https://repo.spring.io/snapshot</url>
            <releases>
                <enabled>false</enabled>
            </releases>
        </pluginRepository>
    </pluginRepositories>

</project>

编写项目的场景启动器：

package com.rainbowsea;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Mp03Application {

    public static void main(String[] args) {
        SpringApplication.run(Mp03Application.class, args);
    }

}

创建的数据库表和结构。如下：

编写对应 数据表的 Java Bean 类对象。

package com.rainbowsea.bean;


import com.baomidou.mybatisplus.annotation.TableField;
import com.baomidou.mybatisplus.annotation.TableName;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@NoArgsConstructor
@AllArgsConstructor
public class User {

    private Long id;
    //@TableField(value = "username")
    private String name;

    //@TableField(select = false)  // 查询时，不对age 字段进行查询
    private Integer age;
    private String email;

    @TableField(value = "`desc`") // 注意：有两层，但最里面的哪个不是引号
    private String desc;


    @TableField(exist = false)  // 表示// ，不让该 online 属性，作为 SQL语句当中的查询字段
    private Integer online;

}

在 resources 类路径下，创建一个名为 application.yaml 的yaml文件，编写连接数据库的信息内容。

spring:
  datasource:
    driver-class-name: com.mysql.cj.jdbc.Driver
    url: jdbc:mysql://localhost:3306/mybatisplus?serverTimezone=UTC&characterEncoding=utf8&useUnicode=true&useSSL=false
    username: root
    password: MySQL123
  main:
    banner-mode: off  #关闭 spring boot  在命令行当中的图标显示


mybatis-plus:
  global-config:
    banner: false  # 关闭 mybatis-plus 在命令行当中的图标显示
    db-config:
      table-prefix: rainbowsea_ # 还可以通过统一添加前缀的方式：

  configuration:
    log-impl: org.apache.ibatis.logging.stdout.StdOutImpl  # 开启 Log 日志信息打印
    map-underscore-to-camel-case: true # 开启驼峰，下划线映射规则

编写配置类，通过配置类的方式，将数据库连接池换成为 Druid 数据库连接池。

package com.rainbowsea.config;


import com.alibaba.druid.pool.DruidDataSource;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

import javax.sql.DataSource;

@Configuration  // 标注配置类
public class DruidDataSourceConfig {

    @Bean
    @ConfigurationProperties(value = "spring.datasource")
    public DataSource getDataSource() {
        DruidDataSource druidDataSource = new DruidDataSource();

        return druidDataSource;
    }
}

运行测试：看看是否切换为了 Druid 数据库连接池

package com.rainbowsea;

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.jdbc.core.JdbcTemplate;

import javax.annotation.Resource;

@SpringBootTest
class Mp03ApplicationTests {


    @Resource
    //@Autowired
    private JdbcTemplate jdbcTemplate;

    @Test
    void getDataSourceTest() {
        System.out.println(jdbcTemplate.getDataSource().getClass());
    }

}

编写：该User表的 mapper 方法。

package com.rainbowsea.mapper;

import com.baomidou.mybatisplus.core.mapper.BaseMapper;
import com.rainbowsea.bean.User;
import org.apache.ibatis.annotations.Mapper;


@Mapper  // 包扫描路径
public interface UserMapper extends BaseMapper<User> {


}

3. 等值查询

3.1 eq (条件筛选属性 = ?)

使用QueryWrapper对象，构建查询条件

等值查询使用：userQueryWrapper.eq()

default Children eq(R column, Object val) {
   return eq(true, column, val);
}

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.HashMap;
import java.util.List;


@SpringBootTest  // 注意这个测试必须要有一个场景类才行，不然是无法运行的。测试的类不同的话，还需要指明
// 包
public class QueryTest {
    @Autowired
    private UserMapper userMapper;

    @Test
    void eq() {
        // 1.创建条件查询对象
        QueryWrapper<User> userQueryWrapper = new QueryWrapper<>();
        // 2. 设置查询条件，指定查询的字段和匹配的值
        QueryWrapper<User> eq = userQueryWrapper.eq("name", "Jone");

        // 3. 进行条件查询
        User user = userMapper.selectOne(eq);
        System.out.println(user);
    }
}

我们思考如果每次都是自己进行字段名称的编写，有可能会出现名称写错的情况，怎么避免这种情况呢，我们可以使用LambdaQueryWrapper对象，在构建字段时，使用方法引用的方式来选择字段，这样做可以避免字段拼写错误出现问题。

代码如下：

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.HashMap;
import java.util.List;


@SpringBootTest  // 注意这个测试必须要有一个场景类才行，不然是无法运行的。测试的类不同的话，还需要指明
// 包
public class QueryTest {
    @Autowired
    private UserMapper userMapper;

    // 使用: LambdaQueryWrapper 进行引用查询，防止报错
    @Test
    void eq2() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        LambdaQueryWrapper<User> queryWrapper = lambdaQueryWrapper.eq(User::getName, "Jone");

        User user = userMapper.selectOne(queryWrapper);
        System.out.println(user);
    }
}

还要考虑一种情况，我们构建的条件是从哪里来的？应该是从客户端通过请求发送过来的，

由服务端接收的。在网站中一般都会有多个条件入口，用户可以选择一个或多个条件进行查询，那这个时候在请求时，我们不能确定所有的条件都是有值的，部分条件可能用户没有传值，那该条件就为null。

​ 比如在电商网站中，可以选择多个查询条件。

那为null的条件，我们是不需要进行查询条件拼接的，否则就会出现如下情况，将为null的条件进行拼接，筛选后无法查询出结果。

当然我们要解决这个问题，可以先判断是否为空，根据判断结果选择是否拼接该字段，这个功能其实不需要我们写，由MybatisPlus的方法已经提供好了。

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.HashMap;
import java.util.List;


@SpringBootTest  // 注意这个测试必须要有一个场景类才行，不然是无法运行的。测试的类不同的话，还需要指明
// 包
public class QueryTest {
    @Autowired
    private UserMapper userMapper;

    // 字段名为 不null的
    @Test
    void isNull2() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        String name = null;
        //String name = "Jone";

        //  public Children eq(boolean condition, R column, Object val) {
        lambdaQueryWrapper.eq(name != null, User::getName, name);
        //User user = userMapper.selectOne(lambdaQueryWrapper);
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);

    }
}

3.2 allEq(满足多个条件字段的值的筛选)

先演示一下如何通过多个eq,构建多条件查询。

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.HashMap;
import java.util.List;


@SpringBootTest  // 注意这个测试必须要有一个场景类才行，不然是无法运行的。测试的类不同的话，还需要指明
// 包
public class QueryTest {

    @Autowired
    private UserMapper userMapper;

    // and 满足多个条件
    @Test
    void allEql() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.eq(User::getName, "Tom");
        lambdaQueryWrapper.eq(User::getAge, 18);
        User user = userMapper.selectOne(lambdaQueryWrapper);
        System.out.println(user);
    }
}

如果此时有多个条件需要同时判断，我们可以将这多个条件放入到Map集合中，更加的方便

allEq(Map<R, V> params, boolean null2IsNull)
参数params:表示传递的Map集合
参数null2IsNull:表示对于为null的条件是否判断isNull

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.HashMap;
import java.util.List;


@SpringBootTest  // 注意这个测试必须要有一个场景类才行，不然是无法运行的。测试的类不同的话，还需要指明
// 包
public class QueryTest {

    @Autowired
    private UserMapper userMapper;

    // 属性值为 null 的查询，属性值不为 Null的不查询该字段
    @Test
    void allEq2() {

        HashMap<String, Object> hashMap = new HashMap<>();

        hashMap.put("name", "Tom");
        hashMap.put("age",18);
        //hashMap.put("age", null);  // 为null 的属性值，则会不查询

        QueryWrapper<User> userQueryWrapper = new QueryWrapper<>();
        //userQueryWrapper.allEq(hashMap,false); //  // 为null 的属性值，则会不查询
        userQueryWrapper.allEq(hashMap, true);  // 为null,(name = ? AND age IS NULL) 查询

        User user = userMapper.selectOne(userQueryWrapper);
        System.out.println(user);
    }
}

3.3 ne (不等于)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.HashMap;
import java.util.List;


@SpringBootTest  // 注意这个测试必须要有一个场景类才行，不然是无法运行的。测试的类不同的话，还需要指明
// 包
public class QueryTest {

    @Autowired
    private UserMapper userMapper;

    // 不等于数值的
    @Test
    void ne() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.ne(User::getName, "Tom");
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);

    }
}

4. 范围查询

4.1 gt( > 大于的范围)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.HashMap;
import java.util.List;


@SpringBootTest  // 注意这个测试必须要有一个场景类才行，不然是无法运行的。测试的类不同的话，还需要指明
// 包
public class QueryTest {

    @Autowired
    private UserMapper userMapper;

    // > 查询
    @Test
    void gt() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        Integer age = 18;
        lambdaQueryWrapper.gt(User::getAge, age);
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

4.2 ge(>= 大于等于的范围)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.HashMap;
import java.util.List;


@SpringBootTest  // 注意这个测试必须要有一个场景类才行，不然是无法运行的。测试的类不同的话，还需要指明
// 包
public class QueryTest {

    @Autowired
    private UserMapper userMapper;

    // >=查询
    @Test
    void ge() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        Integer age = 18;
        lambdaQueryWrapper.ge(User::getAge, age);
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

4.3 lt(< 小于的范围)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.HashMap;
import java.util.List;


@SpringBootTest  // 注意这个测试必须要有一个场景类才行，不然是无法运行的。测试的类不同的话，还需要指明
// 包
public class QueryTest {

    @Autowired
    private UserMapper userMapper;


    // <
    @Test
    void lt() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        Integer age = 20;
        lambdaQueryWrapper.lt(User::getAge, age);
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

4.4 le(小于等于的范围)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.HashMap;
import java.util.List;


@SpringBootTest  // 注意这个测试必须要有一个场景类才行，不然是无法运行的。测试的类不同的话，还需要指明
// 包
public class QueryTest {

    @Autowired
    private UserMapper userMapper;

    // <=
    @Test
    void le() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        Integer age = 20;
        LambdaQueryWrapper<User> lambdaQueryWrapper1 = lambdaQueryWrapper.le(User::getAge, age);

        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

4.5 between(在该区间的范围)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.HashMap;
import java.util.List;


@SpringBootTest  // 注意这个测试必须要有一个场景类才行，不然是无法运行的。测试的类不同的话，还需要指明
// 包
public class QueryTest {

    @Autowired
    private UserMapper userMapper;

    // 范围之间
    @Test
    void between() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.between(User::getAge, 10, 20);
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);

    }
}

4.6 notBetween(不在该区间的范围)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.HashMap;
import java.util.List;


@SpringBootTest  // 注意这个测试必须要有一个场景类才行，不然是无法运行的。测试的类不同的话，还需要指明
// 包
public class QueryTest {

    @Autowired
    private UserMapper userMapper;

    // 不在范围的：  (age NOT BETWEEN ? AND ?)
    @Test
    void notBetween() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.notBetween(User::getAge,10,18);
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

5. 模糊查询

5.1 like( %值%)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.HashMap;
import java.util.List;


@SpringBootTest  // 注意这个测试必须要有一个场景类才行，不然是无法运行的。测试的类不同的话，还需要指明
// 包
public class QueryTest {

    @Autowired
    private UserMapper userMapper;

    // 模糊查询: %J%(String)
    @Test
    void like() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.like(User::getName,"J");
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

5.2 notLike(不满足 %值%)

5.3 likeLeft（%值）

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.HashMap;
import java.util.List;


@SpringBootTest  // 注意这个测试必须要有一个场景类才行，不然是无法运行的。测试的类不同的话，还需要指明
// 包
public class QueryTest {

    @Autowired
    private UserMapper userMapper;

    // 模糊查询:  %e(String) 左边
    @Test
    void likeft() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.likeLeft(User::getName,"e");
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);

    }
}

5.4 likeRight(值%)

6. 判空查询

6.1 isNUll(判断是否为 Null )

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.boot.test.context.SpringBootTest;

import javax.annotation.Resource;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

@SpringBootTest
public class QueryTest02 {
    @Resource
   private UserMapper userMapper;

    // 判断是否为 null  WHERE (name IS NULL)
    @Test
    void isNUll3() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.isNull(User::getName);
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

6.2 isNotNull

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.boot.test.context.SpringBootTest;

import javax.annotation.Resource;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

@SpringBootTest
public class QueryTest02 {
    @Resource
    private UserMapper userMapper;

    //  WHERE (name IS NULL)
    @Test
    void isNotNull() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.isNotNull(User::getName);
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

7. 包含查询

7.1 in（包含该内容的字段）

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.boot.test.context.SpringBootTest;

import javax.annotation.Resource;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

@SpringBootTest
public class QueryTest02 {
    @Resource
    private UserMapper userMapper;

    // 字段 = 值 or 字段 = 值 ->in
    // r WHERE (age IN (?,?,?))
    @Test
    void in() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        ArrayList<Object> arrayList = new ArrayList<>();
        Collections.addAll(arrayList, 18, 20, 22);
        lambdaQueryWrapper.in(User::getAge, arrayList);
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);

    }
}

7.2 notIn(不包含该内容的字段)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.boot.test.context.SpringBootTest;

import javax.annotation.Resource;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

@SpringBootTest
public class QueryTest02 {
    @Resource
    private UserMapper userMapper;

    // 字段！=值 and 字段！=值 ->not in
    //  WHERE (age NOT IN (?,?,?))
    @Test
    void notIn() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        ArrayList<Integer> arrayList = new ArrayList<>();
        Collections.addAll(arrayList, 18, 20, 22);
        lambdaQueryWrapper.notIn(User::getAge, arrayList);
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

7.3 inSql(包含该内容的字段)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.boot.test.context.SpringBootTest;

import javax.annotation.Resource;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

@SpringBootTest
public class QueryTest02 {
    @Resource
    private UserMapper userMapper;

    // er WHERE (age IN (18,20,22))
    @Test
    void inSql() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.inSql(User::getAge, "18,20,22");
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

7.4 notInSql(不包含该内容的字段)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.boot.test.context.SpringBootTest;

import javax.annotation.Resource;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

@SpringBootTest
public class QueryTest02 {
    @Resource
    private UserMapper userMapper;

    // age NOT IN (18,20,22))
    @Test
    void notInsql() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.notInSql(User::getAge, "18,20,22");
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

---

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.boot.test.context.SpringBootTest;

import javax.annotation.Resource;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

@SpringBootTest
public class QueryTest02 {
    @Resource
    private UserMapper userMapper;


    // RE (age NOT IN (select age from rainbowsea_user where age > 20))
    @Test
    void notInSql2() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.notInSql(User::getAge, "select age from rainbowsea_user where age > 20");
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

8. 分组查询

8.1 groupBy(分组的字段)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.awt.*;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;

@SpringBootTest
public class QueryTest03 {

    @Autowired
    private UserMapper userMapper;

    // select age,count(*) as field_count from rainbowsea_user group by age;
    // 分组查询
    @Test
    void groupBy() {
        QueryWrapper<User> queryWrapper = new QueryWrapper<>();
        // 分组字段
        queryWrapper.groupBy("age");
        // 查询字段
        queryWrapper.select("age,count(*) as field_count");
        List<Map<String, Object>> maps = userMapper.selectMaps(queryWrapper);
        System.out.println(maps);
    }
}

9. 聚合查询

9.1 having(聚合的字段)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.awt.*;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;

@SpringBootTest
public class QueryTest03 {

    @Autowired
    private UserMapper userMapper;

    // select age,count(*) as field_count from rainbowsea_user group by age HAVING field_count >=2;
    // 聚合查询
    @Test
    void having() {
        QueryWrapper<User> userQueryWrapper = new QueryWrapper<>();
        // 查询字段
        userQueryWrapper.select("age,count(*) as field_count");
        // 聚合条件筛选
        userQueryWrapper.having("field_count = 1");
        List<Map<String, Object>> maps = userMapper.selectMaps(userQueryWrapper);
        System.out.println(maps);
    }
}

10. 排序查询

10.1 orderByAsc(升序)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.awt.*;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;



@SpringBootTest
public class QueryTest03 {

    @Autowired
    private UserMapper userMapper;

    // 降序
    @Test
    void orderByAsc() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.orderByAsc(User::getAge, User::getId);
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

10.2 orderByDesc(降序)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.awt.*;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;



@SpringBootTest
public class QueryTest03 {

    @Autowired
    private UserMapper userMapper;

    // 升序
    @Test
    void orderByDesc() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.orderByDesc(User::getAge, User::getId);
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

10.3 orderBy(多字段排序定义)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.awt.*;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;





@SpringBootTest
public class QueryTest03 {

    @Autowired
    private UserMapper userMapper;

    @Test
    void orderBy() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        // 设置排序字段和排序的方式
        // 参数1：如果排序字段的值为null的时候，是否还要作为排序字段参与排序
        // 参数2：是否升序排序，
        // 参数3: 排序字段
        //lambdaQueryWrapper.orderBy(true, true, User::getAge);
        lambdaQueryWrapper.orderBy(false, true, User::getAge);
        lambdaQueryWrapper.orderBy(true, false, User::getId);
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

11. func 查询

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.awt.*;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;



@SpringBootTest
public class QueryTest03 {

    @Autowired
    private UserMapper userMapper;

    //   // 可能会根据不同的请情况选择拼接不同的查询条件
    @Test
    void func() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();

        // 可能会根据不同的请情况选择拼接不同的查询条件
      /*  lambdaQueryWrapper.func(new Consumer<LambdaQueryWrapper<User>>() {
            @Override
            public void accept(LambdaQueryWrapper<User> userLambdaQueryWrapper) {
                //if (true) {
                if (false) {
                    userLambdaQueryWrapper.eq(User::getId,1);
                } else {
                    userLambdaQueryWrapper.ne(User::getId,1);
                }
            }
        });*/

        // 使用lambad表达式
        lambdaQueryWrapper.func(userLambdaQueryWrapper -> {
            if (false) {
                userLambdaQueryWrapper.eq(User::getId, 1);
            } else {
                userLambdaQueryWrapper.ne(User::getId, 1);
            }
        });
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

12. 逻辑查询

12.1 and(与)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.awt.*;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;



@SpringBootTest
public class QueryTest03 {

    @Autowired
    private UserMapper userMapper;

    //  WHERE (age > ? AND age < ?)
    @Test
    void and() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.gt(User::getAge, 22).lt(User::getAge, 30);
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.awt.*;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;



@SpringBootTest
public class QueryTest03 {

    @Autowired
    private UserMapper userMapper;


    // WHERE (name = ? AND (age > ? OR age < ?))
    @Test
    void add2() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.eq(User::getName, "wang").and(i -> i.gt(User::getAge, 26).or().lt(User::getAge, 22));
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

12.2 or(或)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.awt.*;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;

@SpringBootTest
public class QueryTest03 {

    @Autowired
    private UserMapper userMapper;

    //  WHERE (age < ? AND age > ?)
    @Test
    void or() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.lt(User::getAge, 20).gt(User::getAge, 23);// age < 20 || age >=23
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

12.3 nested(非)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.awt.*;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;

@SpringBootTest
public class QueryTest03 {

    @Autowired
    private UserMapper userMapper;

    //  WHERE ((name = ? AND age <> ?))
    @Test
    void nested() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.nested(i->i.eq(User::getName,"Tom").ne(User::getAge,22));
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

13. 自定义条件查询

13.1 apply(自定义查询条件)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.awt.*;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;

@SpringBootTest
public class QueryTest03 {

    @Autowired
    private UserMapper userMapper;

    // 自定义条件查询
    @Test
    void apply() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.apply("id = 1");
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

14. last 查询(在sql语句的最末尾拼接“字符串”)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.awt.*;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;

@SpringBootTest
public class QueryTest03 {

    @Autowired
    private UserMapper userMapper;
    // 最后添加字符拼接
    // SELECT id,name,age,email,`desc` FROM rainbowsea_user limit 0,2
    @Test
    void last() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.last("limit 0,2");
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }

}

15. exists查询

15.1 exists(存在查询)

@SpringBootTest
public class QueryTest03 {

    @Autowired
    private UserMapper userMapper;

    // SELECT * from rainbowsea_user WHERE EXISTS (select id FROM rainbowsea_user WHERE age = 18);
    // SELECT * from rainbowsea_user WHERE EXISTS (select id FROM rainbowsea_user WHERE age = 10);

    @Test
    void exists() {
        // 1. 创建QueryWrapper对象
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        // 2. 构建查询条件
        //lambdaQueryWrapper.exists("select id FROM rainbowsea_user WHERE age = 18");
        lambdaQueryWrapper.exists("select id FROM rainbowsea_user WHERE age = 10");
        // 3.查询
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

15.2 notExists(不存在查询)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.awt.*;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;

@SpringBootTest
public class QueryTest03 {

    @Autowired
    private UserMapper userMapper;


    //  SELECT * from rainbowsea_user WHERE not EXISTS (select id FROM rainbowsea_user WHERE age = 10);
    @Test
    void notExists() {
        // 1. 创建QueryWrapper对象
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        // 2. 构建查询条件
        //lambdaQueryWrapper.exists("select id FROM rainbowsea_user WHERE age = 18");
        lambdaQueryWrapper.notExists("select id FROM rainbowsea_user WHERE age = 10");
        // 3.查询
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
}

16. 字段查询

16.1 select(字段查询)

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.rainbowsea.bean.User;
import com.rainbowsea.mapper.UserMapper;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.awt.*;
import java.util.List;
import java.util.Map;
import java.util.function.Consumer;

@SpringBootTest
public class QueryTest03 {

    @Autowired
    private UserMapper userMapper;


    @Test
    void select() {
        LambdaQueryWrapper<User> lambdaQueryWrapper = new LambdaQueryWrapper<>();
        lambdaQueryWrapper.select(User::getId,User::getName);
        List<User> users = userMapper.selectList(lambdaQueryWrapper);
        System.out.println(users);
    }
    
}

17. 最后：

“在这个最后的篇章中，我要表达我对每一位读者的感激之情。你们的关注和回复是我创作的动力源泉，我从你们身上吸取了无尽的灵感与勇气。我会将你们的鼓励留在心底，继续在其他的领域奋斗。感谢你们，我们总会在某个时刻再次相遇。”