Join query and subquery

Generally, the implementation of classification list ：

  Link query ： One time query of three-level classification

        advantage ： Just one query , The corresponding speed is faster when displaying the secondary classification according to the primary classification

        shortcoming ： The efficiency of database query is low , The speed of page loading for the first time is also relatively slow

  Subquery ： First, only query the first level classification , The user clicks / Move the mouse to the first level of classification , Dynamic loading of secondary classification

        advantage ： Improve the efficiency of database query , The speed of page loading for the first time is improved

        shortcoming ： You need to connect to the database multiple times

Data table structure ：

encapsulation Category by CategoryVO：（ by list aggregate ）

package com.qfedu.fmmall.entity;

import javax.persistence.Column;
import java.util.List;

/**
 * @Description:
 * @Author : Jerry
 * @create : 2022-06-24 18:00
 */
public class CategoryVO {

    @Override
    public String toString() {
        return "CategoryVO{" +
                "categoryId=" + categoryId +
                ", categoryName='" + categoryName + '\'' +
                ", categoryLevel=" + categoryLevel +
                ", parentId=" + parentId +
                ", categoryIcon='" + categoryIcon + '\'' +
                ", categorySlogan='" + categorySlogan + '\'' +
                ", categoryPic='" + categoryPic + '\'' +
                ", categoryBgColor='" + categoryBgColor +
                '}';
    }

    /**
     *  Primary key   classification id Primary key 
     */
    @Column(name = "category_id")
    private Integer categoryId;

    /**
     *  Category name   Category name 
     */
    @Column(name = "category_name")
    private String categoryName;

    /**
     *  Classification level   Type of classification ,
     1: A big classification 
     2: Secondary classification 
     3: Three levels of classification 
     */
    @Column(name = "category_level")
    private Integer categoryLevel;

    /**
     *  Parent level id  Father id  The upper level depends on id,1 Level classification is 0, The second level and the third level depend on the upper level respectively 
     */
    @Column(name = "parent_id")
    private Integer parentId;

    /**
     *  Icon  logo
     */
    @Column(name = "category_icon")
    private String categoryIcon;

    /**
     *  slogan 
     */
    @Column(name = "category_slogan")
    private String categorySlogan;

    /**
     *  Classification chart 
     */
    @Column(name = "category_pic")
    private String categoryPic;

    /**
     *  The background color 
     */
    @Column(name = "category_bg_color")
    private String categoryBgColor;


    private List<CategoryVO> categories;

    public List<CategoryVO> getCategories() {
        return categories;
    }

    public void setCategories(List<CategoryVO> categories) {
        this.categories = categories;
    }

    /**
     *  Get primary key   classification id Primary key 
     *
     * @return category_id -  Primary key   classification id Primary key 
     */
    public Integer getCategoryId() {
        return categoryId;
    }

    /**
     *  Set primary key   classification id Primary key 
     *
     * @param categoryId  Primary key   classification id Primary key 
     */
    public void setCategoryId(Integer categoryId) {
        this.categoryId = categoryId;
    }

    /**
     *  Get the category name   Category name 
     *
     * @return category_name -  Category name   Category name 
     */
    public String getCategoryName() {
        return categoryName;
    }

    /**
     *  Set classification name   Category name 
     *
     * @param categoryName  Category name   Category name 
     */
    public void setCategoryName(String categoryName) {
        this.categoryName = categoryName;
    }

    /**
     *  Get the classification level   Type of classification ,
     1: A big classification 
     2: Secondary classification 
     3: Three levels of classification 
     *
     * @return category_level -  Classification level   Type of classification ,
    1: A big classification 
    2: Secondary classification 
    3: Three levels of classification 
     */
    public Integer getCategoryLevel() {
        return categoryLevel;
    }

    /**
     *  Set the classification level   Type of classification ,
     1: A big classification 
     2: Secondary classification 
     3: Three levels of classification 
     *
     * @param categoryLevel  Classification level   Type of classification ,
    1: A big classification 
    2: Secondary classification 
    3: Three levels of classification 
     */
    public void setCategoryLevel(Integer categoryLevel) {
        this.categoryLevel = categoryLevel;
    }

    /**
     *  Get the parent level id  Father id  The upper level depends on id,1 Level classification is 0, The second level and the third level depend on the upper level respectively 
     *
     * @return parent_id -  Parent level id  Father id  The upper level depends on id,1 Level classification is 0, The second level and the third level depend on the upper level respectively 
     */
    public Integer getParentId() {
        return parentId;
    }

    /**
     *  Set parent level id  Father id  The upper level depends on id,1 Level classification is 0, The second level and the third level depend on the upper level respectively 
     *
     * @param parentId  Parent level id  Father id  The upper level depends on id,1 Level classification is 0, The second level and the third level depend on the upper level respectively 
     */
    public void setParentId(Integer parentId) {
        this.parentId = parentId;
    }

    /**
     *  Get the icon  logo
     *
     * @return category_icon -  Icon  logo
     */
    public String getCategoryIcon() {
        return categoryIcon;
    }

    /**
     *  Set icon  logo
     *
     * @param categoryIcon  Icon  logo
     */
    public void setCategoryIcon(String categoryIcon) {
        this.categoryIcon = categoryIcon;
    }

    /**
     *  Get slogans 
     *
     * @return category_slogan -  slogan 
     */
    public String getCategorySlogan() {
        return categorySlogan;
    }

    /**
     *  Set up slogans 
     *
     * @param categorySlogan  slogan 
     */
    public void setCategorySlogan(String categorySlogan) {
        this.categorySlogan = categorySlogan;
    }

    /**
     *  Get the classification map 
     *
     * @return category_pic -  Classification chart 
     */
    public String getCategoryPic() {
        return categoryPic;
    }

    /**
     *  Set the classification chart 
     *
     * @param categoryPic  Classification chart 
     */
    public void setCategoryPic(String categoryPic) {
        this.categoryPic = categoryPic;
    }

    /**
     *  Get the background color 
     *
     * @return category_bg_color -  The background color 
     */
    public String getCategoryBgColor() {
        return categoryBgColor;
    }

    /**
     *  Set the background color 
     *
     * @param categoryBgColor  The background color 
     */
    public void setCategoryBgColor(String categoryBgColor) {
        this.categoryBgColor = categoryBgColor;
    }
}

Link query ：（ Demonstrate three-level query ）

categoryMapper Interface ：

package com.qfedu.fmmall.dao;

import com.qfedu.fmmall.entity.Category;
import com.qfedu.fmmall.entity.CategoryVO;
import com.qfedu.fmmall.general.GeneralDAO;
import org.springframework.stereotype.Repository;

import java.util.List;
@Repository
public interface CategoryMapper extends GeneralDAO<Category> {
    //1. Use connection query to realize classification query ：3 Level query 
    public List<CategoryVO> selectAllCategories();

    //2. Subquery ： according to parentId Query subcategory 
    public List<CategoryVO> selectAllCategories2(int parentId);
}

Mapper file ：

<resultMap id="categoryVOMap" type="com.qfedu.fmmall.entity.CategoryVO">
    <result column="category_id1" jdbcType="INTEGER" property="categoryId" />
    <result column="category_name1" jdbcType="VARCHAR" property="categoryName" />
    <result column="category_level1" jdbcType="INTEGER" property="categoryLevel" />
    <result column="parent_id1" jdbcType="INTEGER" property="parentId" />
    <result column="category_icon1" jdbcType="VARCHAR" property="categoryIcon" />
    <result column="category_slogan1" jdbcType="VARCHAR" property="categorySlogan" />
    <result column="category_pic1" jdbcType="VARCHAR" property="categoryPic" />
    <result column="category_bg_color1" jdbcType="VARCHAR" property="categoryBgColor" />
    <collection property="categories" ofType="com.qfedu.fmmall.entity.CategoryVO">
      <result column="category_id2" jdbcType="INTEGER" property="categoryId" />
      <result column="category_name2" jdbcType="VARCHAR" property="categoryName" />
      <result column="category_level2" jdbcType="INTEGER" property="categoryLevel" />
      <result column="parent_id2" jdbcType="INTEGER" property="parentId" />
      <collection property="categories" ofType="com.qfedu.fmmall.entity.CategoryVO">
        <result column="category_id3" jdbcType="INTEGER" property="categoryId" />
        <result column="category_name3" jdbcType="VARCHAR" property="categoryName" />
        <result column="category_level3" jdbcType="INTEGER" property="categoryLevel" />
        <result column="parent_id3" jdbcType="INTEGER" property="parentId" />
      </collection>
    </collection>
 </resultMap>
<select id="selectAllCategories" resultMap="categoryVOMap">
    select
      c1.category_id 'category_id1',
      c1.category_name 'category_name1',
      c1.category_level 'category_id1',
      c1.parent_id 'parent_id1',
      c1.category_icon 'category_icon1',
      c1.category_slogan 'category_slogan1',
      c1.category_pic 'category_pic1',
      c1.category_bg_color 'category_bg_color1',
      c2.category_id 'category_id2',
      c2.category_name 'category_name2',
      c2.category_level 'category_level2',
      c2.parent_id 'parent_id2',
      c3.category_id 'category_id3',
      c3.category_name 'category_name3',
      c3.category_level 'category_level3',
      c3.parent_id 'parent_id3'
    from category c1
    left join category c2 on c2.parent_id=c1.category_id
    left join category c3 on c3.parent_id=c2.category_id
    where c1.category_level=1
  </select>

test ：

package com.qfedu;

import com.qfedu.fmmall.dao.CategoryMapper;
import com.qfedu.fmmall.entity.CategoryVO;
import org.junit.jupiter.api.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

import java.util.List;

@RunWith(SpringRunner.class)
@SpringBootTest(classes = ApiApplication.class)
class ApiApplicationTests {

    @Autowired
    private CategoryMapper categoryMapper;
    @Test
    void contextLoads() {
        List<CategoryVO> categoryVOS = categoryMapper.selectAllCategories();
        for(CategoryVO c1 : categoryVOS){
            System.out.println(c1);
            for(CategoryVO c2 : c1.getCategories()){
                System.out.println("\t" + c2);
                for(CategoryVO c3 : c2.getCategories()){
                    System.out.println("\t\t" + c3);
                }
            }
        }
    }

}

test result ：

Subquery ：

categoryMapper Interface ：

package com.qfedu.fmmall.dao;

import com.qfedu.fmmall.entity.Category;
import com.qfedu.fmmall.entity.CategoryVO;
import com.qfedu.fmmall.general.GeneralDAO;
import org.springframework.stereotype.Repository;

import java.util.List;
@Repository
public interface CategoryMapper extends GeneralDAO<Category> {
    //1. Use connection query to realize classification query ：3 Level query 
    public List<CategoryVO> selectAllCategories();

    //2. Subquery ： according to parentId Query subcategory 
    public List<CategoryVO> selectAllCategories2(int parentId);
}

Mapper file ：

<resultMap id="categoryVOMap2" type="com.qfedu.fmmall.entity.CategoryVO">
    <result column="category_id" jdbcType="INTEGER" property="categoryId" />
    <result column="category_name" jdbcType="VARCHAR" property="categoryName" />
    <result column="category_level" jdbcType="INTEGER" property="categoryLevel" />
    <result column="parent_id" jdbcType="INTEGER" property="parentId" />
    <result column="category_icon" jdbcType="VARCHAR" property="categoryIcon" />
    <result column="category_slogan" jdbcType="VARCHAR" property="categorySlogan" />
    <result column="category_pic" jdbcType="VARCHAR" property="categoryPic" />
    <result column="category_bg_color" jdbcType="VARCHAR" property="categoryBgColor" />
    <collection property="categories" column="category_id" select="com.qfedu.fmmall.dao.CategoryMapper.selectAllCategories2"/>
  </resultMap>

<!-- Classified by parent id Query sub level classification -->
  <select id="selectAllCategories2" resultMap="categoryVOMap2">
    select
      category_id,
      category_name,
      category_level,
      parent_id,
      category_icon,
      category_slogan,
      category_pic,
      category_bg_color
    from category
    where parent_id=#{parentId}
  </select>

test ：

package com.qfedu;

import com.qfedu.fmmall.dao.CategoryMapper;
import com.qfedu.fmmall.entity.CategoryVO;
import org.junit.jupiter.api.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;

import java.util.List;

@RunWith(SpringRunner.class)
@SpringBootTest(classes = ApiApplication.class)
class ApiApplicationTests {

    @Autowired
    private CategoryMapper categoryMapper;
    @Test
    void contextLoads() {
        List<CategoryVO> categoryVOS = categoryMapper.selectAllCategories2(0);
        for(CategoryVO c1 : categoryVOS){
            System.out.println(c1);
            for(CategoryVO c2 : c1.getCategories()){
                System.out.println("\t" + c2);
                for(CategoryVO c3 : c2.getCategories()){
                    System.out.println("\t\t" + c3);
                }
            }
        }
    }

}

test result ：

  summary ：

         Sub query is better for large data , The query efficiency of the database is high ; It is better to use continuous query when there is less data , All data can be queried at one time .

         Continuous queries and subqueries work the same , Start with the first level query , So as to query , But the actual combat effect is different , The two methods can be used for reference .