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

    com.GitHub.yulichang    mybatis-plus-join    1.2.4    com.baomidou    mybatis-plus-boot-starter    3.5.1

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

@Configurationpublic class MybatisPlusConfig {        @Bean    public MybatisPlusInterceptor mybatisPlusInterceptor() {        MybatisPlusInterceptor interceptor = new MybatisPlusInterceptor();        //分页插件        interceptor.addInnerInterceptor(new PaginationInnerInterceptor(DbType.Mysql));        return interceptor;    }        @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 getMethodList(Class mapperClass) {        //将原来的保持        List 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

@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;}

@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;}

@Data@NoArgsConstructor@AllArgsConstructorpublic class ProductDTO implements Serializable {    private static final long serialVersionUID = -2281333877153304329L;    private Long id;    private String type;    private String name;    private Double price;}

2、Mapper

public interface OpProductInfoMapper extends MPJBaseMapper {}

public interface OpProductMapper extends MPJBaseMapper {}

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 {    List queryAllProduct();}

@Service@Slf4j@AllArgsConstructorpublic class OpProductServiceImpl extends MPJBaseServiceImpl implements OpProductService {    @Resource    private OpProductMapper opProductMapper;    @Override    public List queryAllProduct() {        MPJLambdaWrapper mpjLambdaWrapper = new MPJLambdaWrapper()                .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 list = opProductMapper.selectJoinList(ProductDTO.class, mpjLambdaWrapper);        return list;    }}

4、测试

@SpringBootTest@Slf4jpublic class MybatisJoinTests {    @Autowired    private OpProductService opProductService;        @Test    void test1() {        List 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 list = xxxxxMapper.selectJoinList(xxxxx.class,     new MPJqueryWrapper()      .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()                .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 list = opProductMapper.selectJoinList(ProductDTO.class, mpjLambdaWrapper);        return list;

2、分页代码举例

    public IPage queryPageProduct(Integer pageNo, Integer pageCount) {        MPJLambdaWrapper mpjLambdaWrapper = new MPJLambdaWrapper()                .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 page = opProductMapper.selectJoinPage(new Page(pageNo, pageCount), ProductDTO.class, mpjLambdaWrapper);        return page;    }

来源地址：https://blog.csdn.net/weixin_46146718/article/details/125279384