MyBatis-Plus多表联查的实现方法(动态查询和静态查询)

DROP DATABASE IF EXISTS mp;
CREATE DATABASE mp DEFAULT CHARACTER SET utf8;
USE mp;
 
DROP TABLE IF EXISTS `t_user`;
DROP TABLE IF EXISTS `t_blog`;
SET NAMES utf8mb4;
CREATE TABLE `t_user`
(
    `id`              BIGINT(0) NOT NULL AUTO_INCREMENT,
    `user_name`       VARCHAR(64) NOT NULL COMMENT '用户名（不能重复）',
    `nick_name`       VARCHAR(64) NOT NULL COMMENT '昵称（可以重复）',
    `email`           VARCHAR(64) COMMENT '邮箱',
    `create_time`  DATETIME(0) NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
    `update_time` DATETIME(0) NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '修改时间',
    `deleted_flag` BIGINT(0) NOT NULL DEFAULT 0 COMMENT '0：未删除 其他：已删除',
    PRIMARY KEY (`id`) USING BTREE,
    UNIQUE KEY `index_user_name_deleted_flag`(`user_name`, `deleted_flag`),
    KEY `index_create_time`(`create_time`)
) ENGINE = InnoDB COMMENT = '用户';
CREATE TABLE `t_blog`
    `id`           BIGINT(0) NOT NULL AUTO_INCREMENT,
    `user_id`      BIGINT(0) NOT NULL,
    `user_name`    VARCHAR(64) NOT NULL,
    `title`        VARCHAR(256) CHARACTER SET utf8mb4 NOT NULL COMMENT '标题',
    `description`  VARCHAR(256) CHARACTER SET utf8mb4 NOT NULL COMMENT '摘要',
    `content`      LONGTEXT CHARACTER SET utf8mb4 NOT NULL COMMENT '内容',
    KEY `index_user_id`(`user_id`),
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COMMENT = '博客';
INSERT INTO `t_user` VALUES (1, 'knife', '刀刃', 'abc@qq.com', '2021-01-23 09:33:36', '2021-01-23 09:33:36', 0);
INSERT INTO `t_user` VALUES (2, 'sky', '天蓝', '123@qq.com', '2021-01-24 18:12:21', '2021-01-24 18:12:21', 0);
INSERT INTO `t_blog` VALUES (1, 1, 'knife', 'Java中枚举的用法',
                             '本文介绍Java的枚举类的使用',
                             '2021-01-23 11:33:36', '2021-01-23 11:33:36', 0);
INSERT INTO `t_blog` VALUES (2, 1, 'knife', 'Java中泛型的用法',
                             '本文介绍Java的泛型的使用。',
                             '2021-01-28 23:37:37', '2021-01-28 23:37:37', 0);
INSERT INTO `t_blog` VALUES (3, 1, 'knife', 'Java的HashMap的原理',
                             '本文介绍Java的HashMap的原理。',
                             '2021-05-28 09:06:06', '2021-05-28 09:06:06', 0);
INSERT INTO `t_blog` VALUES (4, 1, 'knife', 'Java中BigDecimal的用法',
                             '本文介绍Java的BigDecimal的使用。',
                             '2021-06-24 20:36:54', '2021-06-24 20:36:54', 0);
INSERT INTO `t_blog` VALUES (5, 1, 'knife', 'Java中反射的用法',
                             '本文介绍Java的反射的使用。',
                             '2021-10-28 22:24:18', '2021-10-28 22:24:18', 0);
INSERT INTO `t_blog` VALUES (6, 2, 'sky', 'Vue-cli的使用',
                             'Vue-cli是Vue的一个脚手架工具',
                             'Vue-cli可以用来创建vue项目',
                             '2021-02-23 11:34:36', '2021-02-25 14:33:36', 0);
INSERT INTO `t_blog` VALUES (7, 2, 'sky', 'Vuex的用法',
                             'Vuex是vue用于共享变量的插件',
                             '一般使用vuex来共享变量',
                             '2021-03-28 23:37:37', '2021-03-28 23:37:37', 0);

<?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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
 
    <groupId>com.example</groupId>
    <artifactId>MyBatis-Plus_Multiple</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>
    <name>MyBatis-Plus_Multiple</name>
    <description>Demo project for Spring Boot</description>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.3.12.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <java.version>1.8</java.version>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <scope>runtime</scope>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
            <groupId>com.baomidou</groupId>
            <artifactId>mybatis-plus-boot-starter</artifactId>
            <version>3.5.1</version>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <groupId>com.github.xiaoymin</groupId>
            <artifactId>knife4j-spring-boot-starter</artifactId>
            <version>3.0.3</version>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <!-- 指定maven编译的jdk版本。若不指定,maven3默认用jdk 1.5 maven2默认用jdk1.3 -->
                <configuration>
                    <source>8</source>
                    <target>8</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

spring:
  datasource:
    driver-class-name: com.mysql.cj.jdbc.Driver
    url: jdbc:mysql://127.0.0.1:3306/mp?serverTimezone=Asia/Shanghai&useUnicode=true&characterEncoding=utf8&useSSL=false
    password: 222333
    username: root
 
#mybatis-plus配置控制台打印完整带参数SQL语句
mybatis-plus:
  configuration:
    log-impl: org.apache.ibatis.logging.stdout.StdOutImpl

package com.example.demo.config;
 
import com.baomidou.mybatisplus.annotation.DbType;
import com.baomidou.mybatisplus.extension.plugins.MybatisPlusInterceptor;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import com.baomidou.mybatisplus.extension.plugins.inner.PaginationInnerInterceptor;
@Configuration
public class MyBatisPlusConfig {
    /**
     * 分页插件
     */
    @Bean
    public MybatisPlusInterceptor mybatisPlusInterceptor() {
        MybatisPlusInterceptor interceptor = new MybatisPlusInterceptor();
        interceptor.addInnerInterceptor(new PaginationInnerInterceptor(DbType.MYSQL));
        return interceptor;
    }
}

package com.example.demo.business.blog.vo;
 
import lombok.Data;
import java.time.LocalDateTime;
@Data
public class BlogVO {
    private Long id;
    private Long userId;
    private String userName;
    /**
     * 标题
     */
    private String title;
     * 摘要
    private String description;
     * 内容
    private String content;
     * 创建时间
    private LocalDateTime createTime;
     * 修改时间
    private LocalDateTime updateTime;
     * 昵称（这个是t_user的字段）
    private String nickName;
}

package com.example.demo.business.blog.mapper;
 
import com.baomidou.mybatisplus.core.conditions.Wrapper;
import com.baomidou.mybatisplus.core.mapper.BaseMapper;
import com.baomidou.mybatisplus.core.metadata.IPage;
import com.example.demo.business.blog.entity.Blog;
import com.example.demo.business.blog.vo.BlogVO;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;
import org.springframework.stereotype.Repository;
import java.util.List;
@Repository
public interface BlogMapper extends BaseMapper<Blog> {
    /**
     * 静态查询
     */
    @Select("SELECT t_user.user_name " +
            " FROM t_blog, t_user " +
            " WHERE t_blog.id = #{id} " +
            "     AND t_blog.user_id = t_user.id")
    String findUserNameByBlogId(@Param("id") Long id);
     * 动态查询
    @Select("SELECT * " +
            " ${ew.customSqlSegment} ")
    IPage<BlogVO> findBlog(IPage<BlogVO> page, @Param("ew") Wrapper wrapper);
}

package com.example.demo.business.blog.controller;
 
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.baomidou.mybatisplus.core.metadata.IPage;
import com.baomidou.mybatisplus.extension.plugins.pagination.Page;
import com.example.demo.business.blog.mapper.BlogMapper;
import com.example.demo.business.blog.vo.BlogVO;
import io.swagger.annotations.Api;
import io.swagger.annotations.ApiOperation;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.util.StringUtils;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
@Api(tags = "自定义SQL")
@RestController
@RequestMapping("/blog")
public class BlogController {
    @Autowired
    private BlogMapper blogMapper;
    @ApiOperation("静态查询")
    @GetMapping("staticQuery")
    public String staticQuery() {
        return blogMapper.findUserNameByBlogId(1L);
    }
    @ApiOperation("动态查询")
    @GetMapping("dynamicQuery")
    public IPage<BlogVO> dynamicQuery(Page<BlogVO> page, String nickName, String title) {
        QueryWrapper<BlogVO> queryWrapper = new QueryWrapper<>();
        queryWrapper.like(StringUtils.hasText(nickName), "t_user.nick_name", nickName);
        queryWrapper.like(StringUtils.hasText(title), "t_blog.title", title);
        queryWrapper.eq("t_blog.deleted_flag", 0);
        queryWrapper.eq("t_user.deleted_flag", 0);
        queryWrapper.apply("t_blog.user_id = t_user.id");
        return blogMapper.findBlog(page, queryWrapper);
}