Mybatis-Plus高级查询LambdaQueryWrapper&QueryWrapper

我刚刚毕业开始进入项目组的时候，从未使用过Mybatis-Plus，只用过Mybatis，Mybatis还是老套的xml配置化，已经快转正了，所以做个总结，现在SpringBoot里面的JPa可以注解实现SQL的增删改查，针对单表查询现在实际项目开发过程中根本不使用原生Hibernate或者Mybatis了，目前使用的是Mybatis-Plus，使用轻便友好，开发代码段少且完美的实现，在这里写一个文档针对我进入工作时是如何开发使用的！

优势：单表查询的话，可以直接的使用对象操作，其实实现起来极其方便而且简单！

如果多表联查的话，一般四张表以上关联或者是很复杂的sql，那就建议使用xml格式的配置化进行关联！

目前在重构一个项目的时候要针对原有的SQL进行重写，所以针对一个好用的Mybatis的插件使用。在这里做一些总结，然后通过我们组内人员使用，统一的改用LambdaQueryWrapper&QueryWrapper

简单对象查询方式用起来极为方便！涉及到单表查询的是该该对象查询继承；

推荐使用：LambdaQueryWrapper

LambdaQueryWrapper使用lambda表达式可以直接通过实体类get()属性，而QueryWrapper必须要与数据库的中表名一致，由于表名可能会很复杂，这时候相较而言LambdaQueryWrapper会比QueryWrapper便捷不少

Wrapper

条件	说明
allEq	基于 map 的比较
eq	等于 =
ne	不等于 <> 或者 !=
gt	大于 >
ge	大于等于 >=
lt	小于 <
le	小于等于 <
between	BETWEEN 值1 AND 值2
notBetween	NOT BETWEEN 值1 AND 值2
like	LIKE ‘%值%’
notLike	NOT LIKE ‘%值%’
likeLeft	LIKE ‘%值’
likeRight	LIKE ‘值%’
isNull	字段 IS NULL
isNotNull	字段 IS NOT NULL
in	字段 IN (value1, value2, …)
notIn	字段 NOT IN (value1, value2, …)
inSql	字段 IN (sql 语句) inSql(“age”, “1,2,3”) -> age in (1,2,3) inSql(“id”, “select id from student where id < 3”) -> id in (select id from student where id < 3)
notInSql	字段 NOT IN (sql 语句)
groupBy	GROUP BY 字段
orderByAsc	升序 ORDER BY 字段, … ASC
orderByDesc	降序 ORDER BY 字段, … DESC
orderBy	自定义字段排序 orderBy(true, true, “id”, “name”) -> order by id ASC, name ASC
having	条件分组
or	OR 语句，拼接 + OR 字段=值
and	AND 语句，拼接 + AND 字段=值
apply	拼接 sql
last	在 sql 语句后拼接自定义条件
exists	拼接 EXISTS(sql语句) exists(“selece id from student where age = 1”) -> exists(selece id from student where age = 1)
notExists	拼接 NOT EXISTS(sql语句)
nested	正常嵌套不带 AND 或者 OR

查询构造器

查询条件

前期准备

创建一个数据库 mybatisplus
创建 user 表

CREATE TABLE `user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(30) DEFAULT NULL,
  `age` int(11) DEFAULT NULL,
  `email` varchar(50) DEFAULT NULL,
  PRIMARY KEY (`id`)
);

创建 springboot 工程
- 导入对应 maven 坐标

<?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.cmy</groupId>
    <artifactId>mybatis_plus</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>mybatis_plus</name>
    <description>Demo project for Spring Boot</description>
    <properties>
        <java.version>1.8</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>

        <!-- mybatis-plus -->
        <dependency>
            <groupId>com.baomidou</groupId>
            <artifactId>mybatis-plus-boot-starter</artifactId>
            <version>3.4.0</version>
        </dependency>

        <!-- mysql -->
        <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>
    </dependencies>

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

</project>

- mysql数据库相关配置

spring:
  datasource:
    driver-class-name: com.mysql.cj.jdbc.Driver
    url: jdbc:mysql://127.0.0.1/mybatisplus?useUnicode=true&characterEncoding=utf-8
    username: root
    password: root

- mybatis-plus 日志信息配置

mybatis-plus:
  configuration:
    log-impl: org.apache.ibatis.logging.stdout.StdOutImpl

- 创建实体类 User

package com.cmy.mybatis_plus.entity;

import com.baomidou.mybatisplus.annotation.IdType;
import com.baomidou.mybatisplus.annotation.TableId;

/**
 * 实体类 user
 */
public class User {
    /**
     * 指定主键id生成的方式
     * value 是主键字段的名称，如果是id，可以不用写
     * type 指定主键的类型，主键的值如何生成。idType.AUTO 自动增长
     */
    @TableId(
            value = "id",
            type = IdType.AUTO
    )
    private Long id;
    private String name;
    private String email;
    private Integer age;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", email='" + email + '\'' +
                ", age=" + age +
                '}';
    }
}

- 自定义 User 的 Mapper 接口

package com.cmy.mybatis_plus.mapper;

import com.baomidou.mybatisplus.core.mapper.BaseMapper;
import com.cmy.mybatis_plus.entity.User;

/**
 * 自定义 Mapper 接口，就是 dao 接口
 * 1. 实现BaseMapper
 * 2. 指定实体类（泛型）
 *
 * BaseMapper 是 MP 框架中的对象，定义了 17 个操作方法（CRUD）
 */
public interface UserMapper extends BaseMapper<User> {
}

查询条件

allEq

条件用 Map 进行封装

“name” -> “张三”

“age” -> 20

public void testAllEq() {
    QueryWrapper<User> queryWrapper = new QueryWrapper<>();
    // 封装条件
    Map<String, Object> hashMap = new HashMap<>();
    hashMap.put("name", "张三");
    hashMap.put("age", 20);

    queryWrapper.allEq(hashMap);

    List<User> userList = userMapper.selectList(queryWrapper);
    userList.forEach(user -> {
        System.out.println(user);
    });
}

eq

eq("列名", 值) -> 列名 = 值

    public List<Dict> listByDictCode(DictCode dictCode) {
        LambdaQueryWrapper<Dict> wrapper = Wrappers.lambdaQuery();
        wrapper.eq(Dict::getDictCode, dictCode.getCode())
               .eq(Dict::getEnabled, DictEnableEnum.VALID.getType());
        return this.baseMapper.selectList(wrapper);
    }

ne

ne("列名", 值) -> 列名 != 值

    public List<Dict> listByDictCode(DictCode dictCode) {
        LambdaQueryWrapper<Dict> wrapper = Wrappers.lambdaQuery();
        wrapper.ne(Dict::getDictCode, dictCode.getCode())
               .ne(Dict::getEnabled, DictEnableEnum.VALID.getType());
        return this.baseMapper.selectList(wrapper);
    }

gt

gt("age", 20) -> age > 20

    public List<User> userList() {
        LambdaQueryWrapper<User> wrapper = Wrappers.lambdaQuery();
        wrapper.gt(User::getAge, 20);            
        return this.baseMapper.selectList(wrapper);
    }

ge

ge("age", 20) -> age >= 20

    public List<User> userList() {
        LambdaQueryWrapper<User> wrapper = Wrappers.lambdaQuery();
        wrapper.ge(User::getAge, 20);            
        return this.baseMapper.selectList(wrapper);
    }

lt

lt("age", 20) -> age < 20

    public List<User> userList() {
        LambdaQueryWrapper<User> wrapper = Wrappers.lambdaQuery();
        wrapper.lt(User::getAge, 20);            
        return this.baseMapper.selectList(wrapper);
    }

le

le("age", 21) -> age <= 21

    public List<User> userList() {
        LambdaQueryWrapper<User> wrapper = Wrappers.lambdaQuery();
        wrapper.le(User::getAge, 20);            
        return this.baseMapper.selectList(wrapper);
    }

between,notBetween

between("age", 18, 25) -> age BETWEEN 18 AND 25 ，年龄在18到25之间

notBetween就是不在18到25之间

    public List<User> userList() {
        LambdaQueryWrapper<User> wrapper = Wrappers.lambdaQuery();
        wrapper.between(User::getAge, 18,25);                
        return this.baseMapper.selectList(wrapper);
    }

like,notLike

like 匹配值 -> "%值%" 模糊查询

notLike 模糊查询不匹配"%值%"

    public List<User> userList() {
        LambdaQueryWrapper<User> wrapper = Wrappers.lambdaQuery();
         wrapper.like(User::getName, "张");           
        return this.baseMapper.selectList(wrapper);
    }

likeLeft

likeLeft 匹配值 -> "%值"

    public List<User> userList() {
        LambdaQueryWrapper<User> wrapper = Wrappers.lambdaQuery();
         wrapper.likeLeft(User::getName, "张");           
        return this.baseMapper.selectList(wrapper);
    }

likeRight

likeRight 匹配值 -> "值%"

    public List<User> userList() {
        LambdaQueryWrapper<User> wrapper = Wrappers.lambdaQuery();
        wrapper.likeRight(User::getName, "张");           
        return this.baseMapper.selectList(wrapper);
    }

isNull 空值查询

isNotNull 非空值查询

    public List<User> userList() {
        LambdaQueryWrapper<User> wrapper = Wrappers.lambdaQuery();
        wrapper.isNull(User::getName);
        //wrapper.isNotNull(User::getName);              
        return this.baseMapper.selectList(wrapper);
    }

in

in("name", "张三", "李四") -> name in ("张三", "李四") 姓名是张三或李四的用户

notIn

notIn("name", "张三", "李四") -> name not in ("张三", "李四") 姓名不是张三或李四的用户

    public List<User> userList() {
        LambdaQueryWrapper<User> wrapper = Wrappers.lambdaQuery();
        wrapper.in(User::getName, "张三","李四");
        //wrapper.in(User::getName, "张三","李四");           
        return this.baseMapper.selectList(wrapper);
    }

inSql、notInSql

public List<User> userList() {
    LambdaQueryWrapper<User> wrapper= new LambdaQueryWrapper<>();
    // SELECT id,name,email,age FROM user WHERE (age IN (select age from user where id = 1))
    wrapper.inSql(User::getAge, "select age from user where id = 1");
    return this.baseMapper.selectList(wrapper);
}

groupBy

分组

public List<User> userList() {
    LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();
    wrapper.groupBy(User::getName);
    return this.baseMapper.selectList(wrapper);
}

orderBy、orderByAsc、orderByDesc

public List<User> userList() {
    LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();
    // SELECT id,name,email,age FROM user ORDER BY name ASC,age DESC
    wrapper.orderBy(true, true, User::getName).orderBy(true, false, User::getAge);
    
    // SELECT id,name,email,age FROM user ORDER BY name ASC,age ASC
    wrapper.orderByAsc(User::getName, User::getAge);

    // SELECT id,name,email,age FROM user ORDER BY name DESC,age DESC
    wrapper.orderByDesc(User::getName, User::getAge);

    return this.baseMapper.selectList(wrapper);
}

or、and

    public List<User> userList() {
        LambdaQueryWrapper<User> wrapper = Wrappers.lambdaQuery();

        // SELECT id,name,email,age FROM user WHERE (name = ? AND id = ?)
        wrapper.eq(User::getName, "张三").and().eq(User::getId,1);

        // SELECT id,name,email,age FROM user WHERE (name = ? OR id = ?)       
        wrapper.eq(User::getName, "张三").or().eq(User::getId,1);     
        return this.baseMapper.selectList(wrapper);
    }

这里说明一下or和and的问题

错误代码

public List<User> userList() {

    LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();
    wrapper.eq(User::getId,1);
    wrapper.like(User::getName,"张")
           .or()
           .like(User::getEmail,"163")
           .or()
           .like(User::getAge,1);
}

根据上面的写法写出的sql语句如下：

WHERE id = '1' 
	AND name LIKE '%张%'
	OR email LIKE '%163%'
    OR age LIKE '%1%'

这样明显是不对的，根据mysql语句执行顺序or最后执行，这会导致一旦[name like '%张%']条件成立后面的or条件就会失效，所以第一个条件并没有起到and的作用。

解决方法

public List<User> userList() {

    LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();
    wrapper.eq(User::getId,1);
    wrapper.and(wrapper->wrapper.like(User::getName,"张")
                                .or()
                                .like(User::getEmail,"163")
                                .or()
                                .like(User::getAge,1)
               );
}

这样得到的sql语句如下

WHERE id = '1' 
	AND (name LIKE '%张%'
	OR email LIKE '%163%'
    OR age LIKE '%1%')

这样就解决了，这个问题在我的公司中新人(包括我在内)貌似都遇到这个问题，在此说明一下

last

在末尾拼接sql语句

注：last()有sql注入的风险，请谨慎使用！

public List<User> userList() {
    LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();
    // SELECT id,name,email,age FROM user WHERE (name = ? OR age = ?) limit 1
    wrapper.eq(User::getName, "张三").or().eq(User::getAge, 20).last("limit 1");
    return this.baseMapper.selectList(wrapper);
}

exists、notExists

public List<User> userList() {
    LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();

    // SELECT id,name,email,age FROM user WHERE (EXISTS (select name from user where age > ?))
    wrapper.exists("select name from user where age > 21");

    // SELECT id,name,email,age FROM user WHERE (NOT EXISTS (select name from user where age > ?))
    wrapper.notExists("select name from user where age > 21");

    return this.baseMapper.selectList(wrapper);
}

总结

略

附加MySQL语句执行顺序

1、from
2、where (or 最后执行)
3、group by
4、having
5、DISTINCT
6、order by
7、limit

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