如何加入一列作为另一列的前缀？

Question

Given two tables containing path columns that indicate the hierarchical placement in an organization, and emp1 that is a member of team1 in department1 in company acme (an emp can have multiple paths), and department managers (one manager can manage several departments):

create table emp (
    username varchar(10),
    path varchar(255)
);

create table manages (
    username varchar(10),
    path varchar(255)
);

insert into emp (username, path)
values ('emp1', '/acme/dept1/team1'),
       ('emp1', '/acme/dept9'),
       ('emp2', '/acme/dept2/team1');

insert into manages (username, path)
values ('mngr1', '/acme/dept1'),
       ('mngr2', '/acme/dept2'),
       ('mngr2', '/acme/dept3');

How can I check if emp1 is a subordinate of mngr1, ie if there is a manages.path that is a prefix of emp.path?

Something like:

select manages.username
from emp
join manages on manages.path is-prefix-of emp.path
where emp.username = 'emp1' and manages.username = 'mngr1'

which should return mngr1.

The actual path implementation is based on a fixed length primary key encoding (meaning that by construction /acme/dept1 wouldn't be a prefix of /acme/dept10).

Answer 1

Use a LIKE expression here:

SELECT m.username
FROM emp e
INNER JOIN manages m
    ON e.path LIKE CONCAT(m.path, '%')
WHERE e.username = 'emp1' AND m.username = 'mngr1';

The LIKE expression in the above query says that e.path starts with m.path .