数据库代码

/*
Navicat MySQL Data Transfer

Source Server         : 本机
Source Server Version : 50546
Source Host           : localhost:3306
Source Database       : springboot

Target Server Type    : MYSQL
Target Server Version : 50546
File Encoding         : 65001

Date: 2023-10-22 16:15:57
*/

SET FOREIGN_KEY_CHECKS=0;

-- ----------------------------
-- Table structure for student
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  `age` int(11) DEFAULT NULL,
  `sex` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=865918979 DEFAULT CHARSET=utf8mb4;

-- ----------------------------
-- Records of student
-- ----------------------------
INSERT INTO `student` VALUES ('1', '张三', '22', '男');
INSERT INTO `student` VALUES ('2', '李四', '32', '男');
INSERT INTO `student` VALUES ('3', '王五', '15', '男');
INSERT INTO `student` VALUES ('4', '樱岛麻衣', '18', '女');
INSERT INTO `student` VALUES ('5', '初音未来', '19', '女');
INSERT INTO `student` VALUES ('865918978', '神里凌华', '17', '女');

0.POM坐标依赖

<?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>2.7.17</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.itheima</groupId>
    <artifactId>springboot_demo04</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>springboot_demo04</name>
    <description>Demo project for Spring Boot</description>
    <properties>
        <java.version>1.8</java.version>
    </properties>
    <dependencies>

        <dependency>
            <groupId>com.baomidou</groupId>
            <artifactId>mybatis-plus-boot-starter</artifactId>
            <version>3.5.1</version>
        </dependency>
        <dependency>
            <groupId>com.alibaba</groupId>
            <artifactId>druid</artifactId>
            <version>1.1.16</version>
        </dependency>

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

        <dependency>
            <groupId>com.mysql</groupId>
            <artifactId>mysql-connector-j</artifactId>
            <scope>runtime</scope>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <optional>true</optional>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <configuration>
                    <excludes>
                        <exclude>
                            <groupId>org.projectlombok</groupId>
                            <artifactId>lombok</artifactId>
                        </exclude>
                    </excludes>
                </configuration>
            </plugin>
        </plugins>
    </build>

</project>

1.创建entity实体层

package com.itheima.entity;

import lombok.Data;

@Data
public class Student {
    private Integer id;
    private String name;
    private Integer age;
    private String sex;
}

2.创建Dao接口层接口,继承BaseMapper<entity>

package com.itheima.dao;

import com.baomidou.mybatisplus.core.mapper.BaseMapper;
import com.itheima.entity.Student;
import org.apache.ibatis.annotations.Mapper;
import org.apache.ibatis.annotations.Select;

import java.util.List;

@Mapper
public interface StudentDao extends BaseMapper<Student> {
    //自定义查询方法
    @Select("SELECT * FROM `student` where age = #{age};")
    List<Student> findByAge(int age);

}

3.创建service接口层，继承IService<entity>

package com.itheima.service;

import com.baomidou.mybatisplus.extension.service.IService;
import com.itheima.entity.Student;

public interface IStudentService extends IService<Student> {
}

4.创建service接口的实现层，继承ServiceImpl<BaseMapper, entity> 并实现IStudentService接口

package com.itheima.service.impl;

import com.baomidou.mybatisplus.extension.service.impl.ServiceImpl;
import com.itheima.dao.StudentDao;
import com.itheima.entity.Student;
import com.itheima.service.IStudentService;
import org.springframework.stereotype.Service;

import java.util.List;

@Service
public class StudnetServiceImpl extends ServiceImpl<StudentDao, Student> implements IStudentService {
    public List<Student> findByAge(int age){
     //调用studentDao中的方法
     return this.baseMapper.findByAge(age);
    }

}

5.编写测试类

package com.itheima;

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.itheima.dao.StudentDao;
import com.itheima.entity.Student;
import com.itheima.service.IStudentService;
import com.itheima.service.impl.StudnetServiceImpl;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.ArrayList;
import java.util.List;

@SpringBootTest
class SpringbootDemo04ApplicationTests {

    @Autowired
    StudentDao student;

    @Autowired
    StudnetServiceImpl studentService;

//    @Autowired
//    Student student1;



    @Test
    void contextLoads() {
        List<Student> students = student.selectList(null);
        System.out.println(students);
    }



    @Test
    //查询年龄小于20岁的
    void selectAgeLt20() {
        LambdaQueryWrapper<Student> lqw = new LambdaQueryWrapper<Student>();
        lqw.lt(Student::getAge, 20);
        List<Student> students = student.selectList(lqw);
        System.out.println(students);

    }

    @Test
    //查询年龄大于30岁的
    void selectAgeGt30(){
        LambdaQueryWrapper<Student> lqw = new LambdaQueryWrapper<Student>();
        lqw.gt(Student::getAge,30);
        System.out.println(student.selectList(lqw));

    }

    @Test
    //查询年龄在20-30岁的
    void selectAge20_30(){
        LambdaQueryWrapper<Student> lqw = new LambdaQueryWrapper<Student>();
        lqw.lt(Student::getAge,30).gt(Student::getAge,20);
        System.out.println(student.selectList(lqw));
    }

    //查询名字中带三的人
    @Test
    void selectLike(){
        LambdaQueryWrapper<Student> lqw = new LambdaQueryWrapper<>();
        lqw.like(Student::getName,'三');
        System.out.println(student.selectList(lqw));

    }

    @Test
    void selectTest(){
        List<Student> list = studentService.list();
        System.out.println(list);
    }

    @Test
    void saveTest(){
        Student student = new Student();
        student.setName("神里凌华");
        student.setAge(17);
        student.setSex("女");
        studentService.saveOrUpdate(student);

    }

    @Test
    void selectByAge(){
        List<Student> list = studentService.findByAge(19);
        System.out.println(list);
    }



}