MyBatis-Plus Tutorial: Creating User Table, Entity, Mapper, and Basic Query Methods

This article provides a step‑by‑step guide for using MyBatis‑Plus in a Java backend project. It covers creating a User table, defining the entity and mapper classes, and performing common query operations.

1. Create User Table

Table structure:

id

name

age

email

1

Jone

18

[email protected]

2

Jack

20

[email protected]

3

Tom

28

[email protected]

4

Sandy

21

[email protected]

5

Billie

24

[email protected]

Corresponding database schema script:

DROP TABLE IF EXISTS user;
CREATE TABLE user (
  id BIGINT(20) NOT NULL COMMENT '主键ID',
  name VARCHAR(30) NULL DEFAULT NULL COMMENT '姓名',
  age INT(11) NULL DEFAULT NULL COMMENT '年龄',
  email VARCHAR(50) NULL DEFAULT NULL COMMENT '邮箱',
  PRIMARY KEY (id)
);

Corresponding data script:

DELETE FROM user;
INSERT INTO user (id, name, age, email) VALUES
  (1, 'Jone', 18, '[email protected]'),
  (2, 'Jack', 20, '[email protected]'),
  (3, 'Tom', 28, '[email protected]'),
  (4, 'Sandy', 21, '[email protected]'),
  (5, 'Billie', 24, '[email protected]');

2. Create Base Classes

2.1 User Entity

package com.example.demo.entity;

import lombok.Data;

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

2.2 UserMapper Interface

package com.example.demo.mapper;

import com.baomidou.mybatisplus.core.mapper.BaseMapper;
import com.example.demo.entity.User;

public interface UserMapper extends BaseMapper<User> {
}

3. Query Methods

3.1 selectList – query all records

@Test
public void testSelect() {
    List<User> userList = userMapper.selectList(null);
    Assert.assertEquals(6, userList.size()); // expects 6 records
    userList.forEach(System.out::println);
}

Corresponding SQL:

SELECT id,name,age,email FROM user

3.2 selectById – query by primary key

@Test
public void testSelectById() {
    User user = userMapper.selectById(2);
    System.out.println(user);
}

Corresponding SQL:

SELECT id,name,age,email FROM user WHERE id=?

3.3 selectBatchIds – batch query by ID list

@Test
public void testSelectByIds() {
    List<Long> idsList = Arrays.asList(2L, 4L, 6L);
    List<User> userList = userMapper.selectBatchIds(idsList);
    userList.forEach(System.out::println);
}

Corresponding SQL:

SELECT id,name,age,email FROM user WHERE id IN ( ?, ?, ? )

3.4 selectByMap – query by conditions

@Test
public void testSelectByMap() {
    Map<String, Object> conditions = new HashMap<>();
    conditions.put("name", "Jack");
    conditions.put("age", 20);
    List<User> userList = userMapper.selectByMap(conditions);
    userList.forEach(System.out::println);
}

Corresponding SQL:

SELECT id,name,age,email FROM user WHERE name = ? AND age = ?

Note: When using selectByMap, the field names in the condition map must exactly match the column names in the database; otherwise an error will occur.

For the full source code and additional details, see the GitHub repository https://github.com/Jackson0714/study-mybatis-plus.git . The article also includes author information and promotional messages.