MySQL represents the child-parent-grandfather relationship

Note: The test database version is MySQL 8.0

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1. Demand

Employee CLARK works for KING, suppose employee CLARK is another employee's manager, what should I do?

select ename,empno,mgr
from emp
where ename in (‘KING’,‘CLARK’,‘MILLER’);
±-------±------±-----+
| ename | empno | mgr |
±-------±------±-----+
| CLARK | 7782 | 7839 |
| KING | 7839 | NULL |
| MILLER | 7934 | 7782 |
±-------±------±-----+

It can be seen that the employee MILLER works for CLARK, and CLARK works for KING.
Now it is necessary to express the complete hierarchy from MILLER to KING.

The following result set is returned:
±----------------------+
| leaf_branch_root |
±----------------- -----+
| MILLER–>CLARK–>KING |
±----------------------+

2. Solution

Perform two self-joins on the table EMP and return to MILLER, CLARK, the manager of MILLER, and KING, the manager of CLARK.

select  concat(a.ename,'-->',b.ename,'-->',c.ename) as leaf_branch_root
  from  emp a, emp b, emp c
 where  a.ename = 'MILLER'
   and  a.mgr = b.empno
   and  b.mgr = c.empno;

Test Record:

mysql> select  concat(a.ename,'-->',b.ename,'-->',c.ename) as leaf_branch_root
    ->   from  emp a, emp b, emp c
    ->  where  a.ename = 'MILLER'
    ->    and  a.mgr = b.empno
    ->    and  b.mgr = c.empno;
+-----------------------+
| leaf_branch_root      |
+-----------------------+
| MILLER-->CLARK-->KING |
+-----------------------+
1 row in set (0.00 sec)