MyBatis-Plus联表查询及分页

---

一、准备工作

mybatis-plus作为mybatis的增强工具，它的出现极大的简化了开发中的数据库操作，但是长久以来，它的联表查询能力一直被大家所诟病。一旦遇到left join或right join的左右连接，你还是得老老实实的打开xml文件，手写上一大段的sql语句。

直到前几天，偶然碰到了这么一款叫做mybatis-plus-join的工具（后面就简称mpj了），使用了一下，不得不说真香！彻底将我从xml地狱中解放了出来，终于可以以类似mybatis-plus中QueryWrapper的方式来进行联表查询了，话不多说，我们下面开始体验。

• mapper继承MPJBaseMapper (必选)
• service继承MPJBaseService (可选)
• serviceImpl继承MPJBaseServiceImpl (可选)

1、数据库结构以及数据

CREATE TABLE `op_product` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `type` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;


INSERT INTO `test_yjdsns`.`op_product`(`id`, `type`) VALUES (1, '苹果');



CREATE TABLE `op_product_info` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `product_id` int(11) NOT NULL,
  `name` varchar(255) DEFAULT NULL,
  `price` decimal(10,2) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

INSERT INTO `test_yjdsns`.`op_product_info`(`id`, `product_id`, `name`, `price`) VALUES (1, 1, '苹果13', 8.00);
INSERT INTO `test_yjdsns`.`op_product_info`(`id`, `product_id`, `name`, `price`) VALUES (2, 1, '苹果15', 9.00);

2、依赖

<dependency>
    <groupId>com.github.yulichanggroupId>
    <artifactId>mybatis-plus-joinartifactId>
    <version>1.2.4version>
dependency>
<dependency>
    <groupId>com.baomidougroupId>
    <artifactId>mybatis-plus-boot-starterartifactId>
    <version>3.5.1version>
dependency>

3、配置类让mybatis-plus-join在DataScopeSqlInjector中生效

/**
 * @author licy
 * @description
 * @date 2022/10/20
 */
@Configuration
public class MybatisPlusConfig {

    /**
     * 新增分页拦截器，并设置数据库类型为mysql
     *
     * @return
     */
    @Bean
    public MybatisPlusInterceptor mybatisPlusInterceptor() {
        MybatisPlusInterceptor interceptor = new MybatisPlusInterceptor();
        //分页插件
        interceptor.addInnerInterceptor(new PaginationInnerInterceptor(DbType.MYSQL));
        return interceptor;
    }

    /**
     * sql注入
     */
    @Bean
    @Primary
    public MySqlInjector myLogicSqlInjector() {
        return new MySqlInjector();
    }

}

修改DataScopeSqlInjector中的继承类为:MPJSqlInjector

public class MySqlInjector extends MPJSqlInjector {

    @Override
    public List<AbstractMethod> getMethodList(Class<?> mapperClass) {
        //将原来的保持
        List<AbstractMethod> methodList = super.getMethodList(mapperClass);
        //多表查询sql注入 从连表插件里移植过来的
        methodList.add(new SelectJoinOne());
        methodList.add(new SelectJoinList());
        methodList.add(new SelectJoinPage());
        methodList.add(new SelectJoinMap());
        methodList.add(new SelectJoinMaps());
        methodList.add(new SelectJoinMapsPage());
        return methodList;
    }
}

4、启动类排除MPJSqlInjector.class

@SpringBootApplication(exclude = {MPJSqlInjector.class})

载入自定义配置类

@Configuration

@MapperScan可以选择tk下的路径

import tk.mybatis.spring.annotation.MapperScan;

二、代码

1、实体类

/**
 * @author licy
 * @description
 * @date 2022/10/25
 */
@Data
@NoArgsConstructor
@AllArgsConstructor
@TableName("op_product")
public class OpProduct implements Serializable {

    private static final long serialVersionUID = -3918932563888251866L;

    @TableId(value = "ID", type = IdType.AUTO)
    private Long id;

    @TableField("TYPE")
    private String type;

}

/**
 * @author licy
 * @description
 * @date 2022/10/25
 */
@Data
@NoArgsConstructor
@AllArgsConstructor
@TableName("op_product_info")
public class OpProductInfo implements Serializable {

    private static final long serialVersionUID = 4186082342917210485L;

    @TableId(value = "ID", type = IdType.AUTO)
    private Long id;

    @TableField("PRODUCT_ID")
    private Long productId;

    @TableField("NAME")
    private String name;

    @TableField("PRICE")
    private Double price;
}

/**
 * @author licy
 * @description
 * @date 2022/10/25
 */
@Data
@NoArgsConstructor
@AllArgsConstructor
public class ProductDTO implements Serializable {

    private static final long serialVersionUID = -2281333877153304329L;

    private Long id;

    private String type;

    private String name;

    private Double price;
}

2、Mapper

/**
 * @author licy
 * @description
 * @date 2022/10/26
 */
public interface OpProductInfoMapper extends MPJBaseMapper<OpProductInfo> {
}

/**
 * @author licy
 * @description
 * @date 2022/10/25
 */
public interface OpProductMapper extends MPJBaseMapper<OpProduct> {
}

3、Service

Mapper接口改造完成后，我们把它注入到Service中，虽然说我们要完成3张表的联表查询，但是以OpProduct作为主表的话，那么只注入这一个对应的OpProductMapper就可以，非常简单。

public interface OpProductService extends MPJBaseService<OpProduct> {

    List<ProductDTO> queryAllProduct();
}

@Service
@Slf4j
@AllArgsConstructor
public class OpProductServiceImpl extends MPJBaseServiceImpl<OpProductMapper, OpProduct> implements OpProductService {

    @Resource
    private OpProductMapper opProductMapper;

    @Override
    public List<ProductDTO> queryAllProduct() {
        MPJLambdaWrapper mpjLambdaWrapper = new MPJLambdaWrapper<ProductDTO>()
                .selectAll(OpProduct.class)//查询表1的全部字段
                .selectAll(OpProductInfo.class)//查询表2的全部字段
                .leftJoin(OpProductInfo.class, OpProductInfo::getProductId, OpProduct::getId);//左查询表2条件为表二的productId=表一的id
        List<ProductDTO> list = opProductMapper.selectJoinList(ProductDTO.class, mpjLambdaWrapper);
        return list;
    }
}

4、测试

@SpringBootTest
@Slf4j
public class MybatisJoinTests {
    @Autowired
    private OpProductService opProductService;
    
    @Test
    void test1() {
        List<ProductDTO> productDTOS = opProductService.queryAllProduct();
        log.info(productDTOS.toString());
    }
}

5、结果

三、分页查询

1、MPJLambdaWrapper几个方法

接下来的MPJLambdaWrapper就是构建查询条件的核心了，看一下我们在上面用到的几个方法：

• selectAll()：查询指定实体类的全部字段
• select()：查询指定的字段，支持可变长参数同时查询多个字段，但是在同一个select中只能查询相同表的字段，所以如果查询多张表的字段需要分开写
• selectAs()：字段别名查询，用于数据库字段与接收结果的dto中属性名称不一致时转换
• leftJoin()：左连接，其中第一个参数是参与联表的表对应的实体类，第二个参数是这张表联表的ON字段，第三个参数是参与联表的ON的另一个实体类属性

除此之外，还可以正常调用mybatis-plus中的各种原生方法，文档中还提到，默认主表别名是t，其他的表别名以先后调用的顺序使用t1、t2、t3以此类推。

和mybatis-plus非常类似，除了LamdaWrapper外还提供了普通QueryWrapper的写法，举例代码：

public void getOrderSimple() {
     List<xxxxxDto> list = xxxxxMapper.selectJoinList(xxxxx.class,
     new MPJQueryWrapper<xxxxx>()
      .selectAll(xxxxx.class)
      .select("t2.unit_price","t2.name as product_name")
      .select("t1.name as user_name")
      .leftJoin("t_user t1 on t1.id = t.user_id")
      .leftJoin("t_product t2 on t2.id = t.product_id")
      .eq("t.status", "3")
    );
    
    log.info(list.toString());
}

或者

        MPJLambdaWrapper mpjLambdaWrapper = new MPJLambdaWrapper<ProductDTO>()
                .selectAll(OpProduct.class)//查询表1的全部字段
                .selectAs(OpProductInfo::getId,"ProductInfoId")//起别名
                .selectAs(OpProductInfo::getName,ProductDTO::getName)//起别名
                .selectAs(OpProductInfo::getPrice,ProductDTO::getPrice)//起别名
                .leftJoin(OpProductInfo.class, OpProductInfo::getProductId, OpProduct::getId);//左查询表2条件为表二的productId=表一的id
        List<ProductDTO> list = opProductMapper.selectJoinList(ProductDTO.class, mpjLambdaWrapper);
        return list;

2、分页代码举例

    public IPage<ProductDTO> queryPageProduct(Integer pageNo, Integer pageCount) {
        MPJLambdaWrapper mpjLambdaWrapper = new MPJLambdaWrapper<ProductDTO>()
                .selectAll(OpProduct.class)//查询表1的全部字段
                .selectAll(OpProductInfo.class)//查询表2的全部字段
                .leftJoin(OpProductInfo.class, OpProductInfo::getProductId, OpProduct::getId);//左查询表2条件为表二的productId=表一的id
        IPage<ProductDTO> page = opProductMapper.selectJoinPage(new Page<ProductDTO>(pageNo, pageCount), ProductDTO.class, mpjLambdaWrapper);
        return page;
    }