MyBatis-Plus怎么实现多表联查

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建库建表

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);

依赖

pom.xml

<?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>

配置

application.yml

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

MyBatis-Plus分页插件配置

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;@Configurationpublic class MyBatisPlusConfig {        @Bean    public MybatisPlusInterceptor mybatisPlusInterceptor() {        MybatisPlusInterceptor interceptor = new MybatisPlusInterceptor();        interceptor.addInnerInterceptor(new PaginationInnerInterceptor(DbType.MYSQL));        return interceptor;    }}

代码

VO

package com.example.demo.business.blog.vo; import lombok.Data;import java.time.LocalDateTime;@Datapublic class BlogVO {    private Long id;    private Long userId;    private String userName;        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;}

Mapper

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;@Repositorypublic interface BlogMapper extends BaseMapper<Blog> {        @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);}

Controller

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);}

测试

访问knife4j页面：http://localhost:8080/doc.html

1.静态查询

2.动态查询 

1.不传条件

结果：（可以查到所有数据） 

后端输出

Creating a new SqlSession
SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@60bbb9ec] was not registered for synchronization because synchronization is not active
JDBC Connection [HikariProxyConnection@1853643659 wrapping com.mysql.cj.jdbc.ConnectionImpl@6a43d29c] will not be managed by Spring
==>  Preparing: SELECT COUNT(*) AS total FROM t_blog, t_user WHERE (t_blog.deleted_flag = ? AND t_user.deleted_flag = ? AND t_blog.user_id = t_user.id)
==> Parameters: 0(Integer), 0(Integer)
<==    Columns: total
<==        Row: 7
<==      Total: 1
==>  Preparing: SELECT * FROM t_blog, t_user WHERE (t_blog.deleted_flag = ? AND t_user.deleted_flag = ? AND t_blog.user_id = t_user.id) LIMIT ?
==> Parameters: 0(Integer), 0(Integer), 10(Long)
<==    Columns: id, user_id, user_name, title, description, content, create_time, update_time, deleted_flag, id, user_name, nick_name, email, create_time, update_time, deleted_flag
<==        Row: 1, 1, knife, Java中枚举的用法, 本文介绍Java的枚举类的使用, <<BLOB>>, 2021-01-23 11:33:36, 2021-01-23 11:33:36, 0, 1, knife, 刀刃, abc@qq.com, 2021-01-23 09:33:36, 2021-01-23 09:33:36, 0
<==        Row: 2, 1, knife, Java中泛型的用法, 本文介绍Java的泛型的使用。, <<BLOB>>, 2021-01-28 23:37:37, 2021-01-28 23:37:37, 0, 1, knife, 刀刃, abc@qq.com, 2021-01-23 09:33:36, 2021-01-23 09:33:36, 0
<==        Row: 3, 1, knife, Java的HashMap的原理, 本文介绍Java的HashMap的原理。, <<BLOB>>, 2021-05-28 09:06:06, 2021-05-28 09:06:06, 0, 1, knife, 刀刃, abc@qq.com, 2021-01-23 09:33:36, 2021-01-23 09:33:36, 0
<==        Row: 4, 1, knife, Java中BigDecimal的用法, 本文介绍Java的BigDecimal的使用。, <<BLOB>>, 2021-06-24 20:36:54, 2021-06-24 20:36:54, 0, 1, knife, 刀刃, abc@qq.com, 2021-01-23 09:33:36, 2021-01-23 09:33:36, 0
<==        Row: 5, 1, knife, Java中反射的用法, 本文介绍Java的反射的使用。, <<BLOB>>, 2021-10-28 22:24:18, 2021-10-28 22:24:18, 0, 1, knife, 刀刃, abc@qq.com, 2021-01-23 09:33:36, 2021-01-23 09:33:36, 0
<==        Row: 6, 2, sky, Vue-cli的使用, Vue-cli是Vue的一个脚手架工具, <<BLOB>>, 2021-02-23 11:34:36, 2021-02-25 14:33:36, 0, 2, sky, 天蓝, 123@qq.com, 2021-01-24 18:12:21, 2021-01-24 18:12:21, 0
<==        Row: 7, 2, sky, Vuex的用法, Vuex是vue用于共享变量的插件, <<BLOB>>, 2021-03-28 23:37:37, 2021-03-28 23:37:37, 0, 2, sky, 天蓝, 123@qq.com, 2021-01-24 18:12:21, 2021-01-24 18:12:21, 0
<==      Total: 7
Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@60bbb9ec]

2.传条件

只传：nickName：刀 

结果

后端结果

Creating a new SqlSession
SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@30026aab] was not registered for synchronization because synchronization is not active
JDBC Connection [HikariProxyConnection@2127441980 wrapping com.mysql.cj.jdbc.ConnectionImpl@6a43d29c] will not be managed by Spring
==>  Preparing: SELECT COUNT(*) AS total FROM t_blog, t_user WHERE (t_user.nick_name LIKE ? AND t_blog.deleted_flag = ? AND t_user.deleted_flag = ? AND t_blog.user_id = t_user.id)
==> Parameters: %刀%(String), 0(Integer), 0(Integer)
<==    Columns: total
<==        Row: 5
<==      Total: 1
==>  Preparing: SELECT * FROM t_blog, t_user WHERE (t_user.nick_name LIKE ? AND t_blog.deleted_flag = ? AND t_user.deleted_flag = ? AND t_blog.user_id = t_user.id) LIMIT ?
==> Parameters: %刀%(String), 0(Integer), 0(Integer), 10(Long)
<==    Columns: id, user_id, user_name, title, description, content, create_time, update_time, deleted_flag, id, user_name, nick_name, email, create_time, update_time, deleted_flag
<==        Row: 1, 1, knife, Java中枚举的用法, 本文介绍Java的枚举类的使用, <<BLOB>>, 2021-01-23 11:33:36, 2021-01-23 11:33:36, 0, 1, knife, 刀刃, abc@qq.com, 2021-01-23 09:33:36, 2021-01-23 09:33:36, 0
<==        Row: 2, 1, knife, Java中泛型的用法, 本文介绍Java的泛型的使用。, <<BLOB>>, 2021-01-28 23:37:37, 2021-01-28 23:37:37, 0, 1, knife, 刀刃, abc@qq.com, 2021-01-23 09:33:36, 2021-01-23 09:33:36, 0
<==        Row: 3, 1, knife, Java的HashMap的原理, 本文介绍Java的HashMap的原理。, <<BLOB>>, 2021-05-28 09:06:06, 2021-05-28 09:06:06, 0, 1, knife, 刀刃, abc@qq.com, 2021-01-23 09:33:36, 2021-01-23 09:33:36, 0
<==        Row: 4, 1, knife, Java中BigDecimal的用法, 本文介绍Java的BigDecimal的使用。, <<BLOB>>, 2021-06-24 20:36:54, 2021-06-24 20:36:54, 0, 1, knife, 刀刃, abc@qq.com, 2021-01-23 09:33:36, 2021-01-23 09:33:36, 0
<==        Row: 5, 1, knife, Java中反射的用法, 本文介绍Java的反射的使用。, <<BLOB>>, 2021-10-28 22:24:18, 2021-10-28 22:24:18, 0, 1, knife, 刀刃, abc@qq.com, 2021-01-23 09:33:36, 2021-01-23 09:33:36, 0
<==      Total: 5
Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@30026aab]

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