MyBatis self-queries and uses recursion to implement N-level linkage

A: First, let’s look at a simple interview question

Fibonacci Sequence

Calculate the array {1,1,2,3,5,8.... ..} The 30th digit value

Rule: 1 1 Starting from the third item, each item is the sum of the previous two items

There are two ways to implement it

The first way:

   TestSelf((n

The second way: using array

 public int TestSelfTwo(int n){       if(n

Formula: f(n) = f(n-2)+f(n-1) f represents the number of bits represented by method n

B: Use recursion to implement n-level linkage in MyBatis

sql statement: select * from type where pid = 0; Specify the pid value as 0 for the first time, and then use the cid with pid as 0 as the next query pid

        public List<category> getCategory(Integer pid);   //接口层方法</category>

Mapping file configuration

<mapper><resultmap><id></id><result></result><collection></collection>   //这里可以不用指定oftype  使用反向查询select从另一个maper文件中取出数据时必须用ofType<!--查到的cid作为下次的pid--></resultmap><select>select * from category where pid=#{pid}</select></mapper>

mybatis’s javaType and ofType

are all types of specified objects. The difference is that when using reverse query select to retrieve data from another maper file, you must use ofType

. The type of object can be specified for collection and association.

is not required to be written, only ofType is required for reverse selection;

Entity class:

package entity;import java.util.HashSet;import java.util.Set;/**
 * Created by zhangyu on 2017/7/12. */public class Category {private Integer  cid;private String cName;private Integer pid;private Set<category> categorySet = new HashSet<category>();
    @Overridepublic String toString() {return "Category{" +
                "cid=" + cid +
                ", cName='" + cName + '\'' +
                ", pid=" + pid +
                ", categorySet=" + categorySet +
                '}';
    }public Integer getCid() {return cid;
    }public void setCid(Integer cid) {this.cid = cid;
    }public String getcName() {return cName;
    }public void setcName(String cName) {this.cName = cName;
    }public Integer getPid() {return pid;
    }public void setPid(Integer pid) {this.pid = pid;
    }public Set<category> getCategorySet() {return categorySet;
    }public void setCategorySet(Set<category> categorySet) {this.categorySet = categorySet;
    }
}</category></category></category></category>

Test class:

 //测试自连接    @Testpublic void  TestSelf(){
        CateGoryDao dao = MyBatis.getSessionTwo().getMapper(CateGoryDao.class);
        List<category> list = dao.getCategory(0);for (Category item:list ) {
            System.out.println(item);
        }
    }</category>

Print result:

Category{cid=1, cName='图书', pid=0, categorySet=[Category{cid=5, cName='期刊报纸', pid=1, categorySet=[]}, Category{cid=3, cName='青年图书', pid=1, categorySet=[Category{cid=6, cName='读者', pid=3, categorySet=[Category{cid=7, cName='12月份', pid=6, categorySet=[]}]}]}, Category{cid=4, cName='少儿图书', pid=1, categorySet=[]}]}
Category{cid=2, cName='服装', pid=0, categorySet=[]}

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