mybtais，是一款优秀的持久层框架，而MybatisPlus则是在mybatis基础上只做增强，不做改变，简称MP，记录一下MP中常用的查询方法。

QueryWrapper与UpdateWrapper共有方法

QueryWrapper特有方法

UpdateWrapper特有方法

实例

package com.xiao.permission_system;

import com.baomidou.mybatisplus.core.conditions.query.LambdaQueryWrapper;
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.baomidou.mybatisplus.extension.service.additional.query.impl.LambdaQueryChainWrapper;
import com.mysql.cj.util.StringUtils;
import com.xiao.permission_system.entity.UserInfo;
import com.xiao.permission_system.mapper.UserInfoMapper;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;;

@RunWith(SpringRunner.class)
@SpringBootTest
public class PermissionSystemApplicationTests {

    @Test
    public void contextLoads() {
    }

    @Autowired
    private UserInfoMapper userInfoMapper;

    /**
     * 普通查询
     */
    @Test
    public void selectById() {
        UserInfo userInfo = userInfoMapper.selectById(123);
        System.out.println(userInfo);
    }

    /**
     * 批量查询
     */
    @Test
    public void selectByIds() {
        List<Long> ids = Arrays.asList(123L,124L,125L);
        List<UserInfo> userInfo = userInfoMapper.selectBatchIds(ids);
        System.out.println(userInfo);
    }

    /**
     * 名字包含娟并且年龄小雨30
     */
    @Test
    public void selectByWrapper() {
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        queryWrapper.like("username","娟").lt("age",30);
        List<UserInfo> userInfoList = userInfoMapper.selectList(queryWrapper);
        userInfoList.forEach(System.out::println);
    }
    /**
     * 名字包含娟并且年龄大雨等于20且小于等于40并且email不为空
     */
    @Test
    public void selectByWrapper2() {
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        queryWrapper.like("username","娟").between("age",20,30).isNotNull("email");
        List<UserInfo> userInfoList = userInfoMapper.selectList(queryWrapper);
        userInfoList.forEach(System.out::println);
    }

    /**
     * 名字姓肖或者年量大雨等于20，按照年龄降序排列，年龄相同按照id生序排列
     */
    @Test
    public void selectByWrapper3() {
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        queryWrapper.likeRight("username","肖")
                .or().ge("age",20).orderByDesc("age").orderByAsc("id");
        List<UserInfo> userInfoList = userInfoMapper.selectList(queryWrapper);
        userInfoList.forEach(System.out::println);
    }

    /**
     * 创建日期为2019年10月2日并且直属上级名字为王姓
     */
    @Test
    public void selectByWrapper4() {
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        queryWrapper.apply("date_format(create_time,'%Y-%m-%d')={0}","2019-10-07")
                .inSql("parent_id","select id from user where username like '王%'");
        List<UserInfo> userInfoList = userInfoMapper.selectList(queryWrapper);
        userInfoList.forEach(System.out::println);
    }

    /**
     * 名字为王姓并且（年龄小于40或邮箱不为空）
     */
    @Test
    public void selectByWrapper5() {
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        //注意：这里的or在and里面
        //参考：https://blog.csdn.net/u011229848/article/details/81902398?utm_source=blogxgwz3
        queryWrapper.likeRight("username","王")
                .and(wq->wq.lt("age",40).or().isNotNull("email"));
        List<UserInfo> userInfoList = userInfoMapper.selectList(queryWrapper);
        userInfoList.forEach(System.out::println);
    }
    /**
     * 名字为王姓并且（年龄小于40并且大与20或邮箱不为空）
     */
    @Test
    public void selectByWrapper6() {
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        queryWrapper.likeRight("username","王")
                .and(wq->wq.lt("age",40).gt("age",20).or().isNotNull("email"));
        List<UserInfo> userInfoList = userInfoMapper.selectList(queryWrapper);
        userInfoList.forEach(System.out::println);
    }
    /**
     * 年龄23，30，40
     */
    @Test
    public void selectByWrapper8() {
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        queryWrapper.in("age",Arrays.asList(20,30,40));
        List<UserInfo> userInfoList = userInfoMapper.selectList(queryWrapper);
        userInfoList.forEach(System.out::println);
    }
    /**
     * 只返回满足条件的第一条语句即可, 用last拼接limit部分
     */
    @Test
    public void selectByWrapper9() {
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        queryWrapper.in("age",Arrays.asList(20,30,40)).last("limit 1");
        List<UserInfo> userInfoList = userInfoMapper.selectList(queryWrapper);
        userInfoList.forEach(System.out::println);
    }

    /**
     * 名字中包含雨并且年龄小于40(只取id,username)
     */
    @Test
    public void selectByWrapper10() {
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        queryWrapper.select("id","username").like("username","雨").lt("age",40);
        List<UserInfo> userInfoList = userInfoMapper.selectList(queryWrapper);
        userInfoList.forEach(System.out::println);
    }

    /**
     * 名字中包含雨并且年龄小于40(不取create_time,parent_id两个字段,即不列出全部字段)
     */
    @Test
    public void selectByWrapper11() {
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        queryWrapper.like("username","雨").lt("age",40)
                    .select(UserInfo.class,info->!info.getColumn().equals("create_time")&&
                            !info.getColumn().equals("parent_id"));
        List<UserInfo> userInfoList = userInfoMapper.selectList(queryWrapper);
        userInfoList.forEach(System.out::println);
    }

    /**
     * 名字为王姓和邮箱不为空，先判断条件是否满足
     */
    public void testCondition() {
        String username = "王";
        String email = "";
        condition(username,email);
    }

    private void condition(String username,String email){
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        queryWrapper.likeRight(StringUtils.isNotBlank(username),"name",username)
                    .like(StringUtils.isNotBlank(email),"email",email);
        List<UserInfo> userInfoList = userInfoMapper.selectList(queryWrapper);
        userInfoList.forEach(System.out::println);
    }

    /**
     * 实体作为条件构造器方法的参数
     */
    @Test
    public void selectByWrapperEntity() {
        UserInfo whereUser = new UserInfo();
        whereUser.setUsername("xiaojuan");
        whereUser.setAge(22);
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>(whereUser);
        List<UserInfo> userInfoList = userInfoMapper.selectList(queryWrapper);
        userInfoList.forEach(System.out::println);
    }
    /**
     * AllEq用法
     */
    @Test
    public void selectByWrapperAllEq() {
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        Map<String, Object> params = new HashMap<String, Object>();
        params.put("nuserame","xiaojuan");
        params.put("age",null);
        queryWrapper.allEq(params);
        List<UserInfo> userInfoList = userInfoMapper.selectList(queryWrapper);
        userInfoList.forEach(System.out::println);
    }

    /**
     * AllEq用法(排除不是条件的字段)
     */
    @Test
    public void selectByWrapperAllEq2() {
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        Map<String, Object> params = new HashMap<String, Object>();
        params.put("nuserame","xiaojuan");
        params.put("age",null);
        queryWrapper.allEq((k,v)->!k.equals("name"),params);
        List<UserInfo> userInfoList = userInfoMapper.selectList(queryWrapper);
        userInfoList.forEach(System.out::println);
    }

    /**
     * selectMaps
     */
    @Test
    public void selectByWrapperMaps() {
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        queryWrapper.like("name","肖").lt("age",40);
        List<Map<String,Object>> userInfoList = userInfoMapper.selectMaps(queryWrapper);
        userInfoList.forEach(System.out::println);
    }

    /**
     * 按照直属上级分组，查询每组的平均年龄，最大年龄，最小年龄。并且只取年龄总和小于500的组
     */
    @Test
    public void selectByWrapperMaps2() {
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        queryWrapper.select("avg(age) avg_age","min(min) min_age","max(age) max_age")
                .groupBy("parent_id").having("sum(age)<{0}",500);
        List<Map<String,Object>> userInfoList = userInfoMapper.selectMaps(queryWrapper);
        userInfoList.forEach(System.out::println);
    }

    /**
     * selectObjs
     */
    @Test
    public void selectByWrapperObjs() {
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        queryWrapper.select("id","name").like("name","肖").lt("age",40);
        List<Object> userInfoList = userInfoMapper.selectObjs(queryWrapper);
        userInfoList.forEach(System.out::println);
    }

    /**
     * selectCount
     */
    @Test
    public void selectByWrapperCount() {
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        queryWrapper.like("name","肖").lt("age",40);
        Integer count = userInfoMapper.selectCount(queryWrapper);
        System.out.println(count);
    }

    /**
     * selectOne
     */
    @Test
    public void selectByWrapperSelectOne() {
        QueryWrapper<UserInfo> queryWrapper = new QueryWrapper<UserInfo>();
        queryWrapper.like("name","肖").lt("age",40);
        UserInfo user = userInfoMapper.selectOne(queryWrapper);
        System.out.println(user);
    }

    /**
     * 使用Lambda
     */
    @Test
    public void selectLambda() {
       // LambdaQueryWrapper<UserInfo> lambda = new QueryWrapper<UserInfo>().lambda();
        LambdaQueryWrapper<UserInfo> lambda = new LambdaQueryWrapper<UserInfo>();
        lambda.like(UserInfo::getUsername,"娟").lt(UserInfo::getAge,40);
        List<UserInfo> userInfoList = userInfoMapper.selectList(lambda);
        userInfoList.forEach(System.out::println);
    }

    /**
     * 使用Lambda,名字为王姓且（年龄小于40或邮箱不为空）
     */
    @Test
    public void selectLambd2() {
        LambdaQueryWrapper<UserInfo> lambda = new LambdaQueryWrapper<UserInfo>();
        lambda.like(UserInfo::getUsername,"娟")
                .and(lqw->lqw.lt(UserInfo::getAge,40).or().isNotNull(UserInfo::getEmail));
        List<UserInfo> userInfoList = userInfoMapper.selectList(lambda);
        userInfoList.forEach(System.out::println);
    }

    /**
     * 使用Lambda链式
     */
    @Test
    public void selectLambd3() {
        List<UserInfo> userInfoList = new LambdaQueryChainWrapper<UserInfo>(userInfoMapper)
                .like(UserInfo::getUsername,"娟").ge(UserInfo::getAge,20).list();
        userInfoList.forEach(System.out::println);
    }
}

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参考：https://www.cnblogs.com/steven0325/p/12463138.html