一、树状结构

MySQL数据库本身并不直接支持树状结构的存储，但它提供了足够的灵活性，允许我们通过不同的方法来模拟和实现树状数据结构。具体方法看下文。

数据库表结构：

实现效果

查询的结果像树一样

 二、使用

以Catalog数据表，findAll()方法举例

2.1 controller

@RestController
@RequestMapping("catalog")
public class CatalogController {
    @Autowired
    private CatalogService catalogService;

    @GetMapping("findAll")
    public Result findAll(){
        return Result.success(catalogService.findAll());
    }
}

2.2service

public interface CatalogService {
    List<Catalog> findAll();
}

@Service
public class CatalogServiceImpl implements CatalogService {

    @Autowired
    private CatalogMapper catalogMapper;
    @Override
    public List<Catalog> findAll() {
//     在这里直接调用findByParentId()方法,等于说mapper的findAll()没用了
        return catalogMapper.findByParentId(0);
    }

}

2.3mapper--有findAll()方法继承了BaseMapper

public interface CatalogMapper extends BaseMapper<Catalog> {
    List<Catalog> findByParentId(Integer parentId);
}

 <collection property="studentCatalog" select="findByParentId" column="id">

利用递归函数，父节点id作为参数，直到没有父节点id结束

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.itqq.mapper.CatalogMapper">

    <resultMap id="catalog" type="Catalog">
        <id property="id" column="id"></id>
        <result property="name" column="name"></result>
        <result property="parentId" column="parent_id"></result>
        <collection property="studentCatalog" select="findByParentId" column="id"></collection>
    </resultMap>
<!--不用也行，可以在CatalogServiceImpl，直接调用findByParentId传id=0就行-->
    <select id="findAll" resultMap="catalog">
        SELECT * FROM catalog where parent_id = 0
    </select>
    <select id="findByParentId" resultMap="catalog">
        select * from catalog where parent_id = #{parentId}
    </select>
</mapper>

2.4pojo

@Data
public class Catalog implements Serializable {
    private static final long serialVersionUID = 152326L;

    private int id;
    private String name;
    private String parentId;

    private List<Catalog> studentCatalog;
}

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总结结束，使用时可以按需修改！！！