Self Referencing SQL query when condition is met - sql

Question

I'm trying to create a SQL query to return the column values from a table that meet certain criteria.
Currently I have used the CONCAT function to join the first and last names into a single column in the query result for employees that have the role of 'Programmer'.
SELECT
person.id, CONCAT(person.firstname,' ', person.lastname) AS FULLNAME
FROM
person, role
WHERE
person.role_id = role.id AND role.name = 'Programmer'
This successfully runs and returns all programmers from the tables. Notice in my table structure I have an actingas_id column. This is the key to another person.id for people who are working on behalf of another people whilst they're on leave from work.
Thus, we arrive at my Question: How do I modify the SQL query such that when a person is acting that the query retrieves the first and last name of this person as well as the person who's 'shoes are being filled'?
My table structure is as follows:
person:
id | firstname | lastname | role_id | actingas_id |
role:
id | name |
+----+-----------+----------+---------+-------------+
| id | firstname | lastname | role_id | actingas_id |
+----+-----------+----------+---------+-------------+
| 1 | John | Smith | 1 | 0 |
| 2 | Kevin | Tull | 2 | 1 |
| 3 | Michael | Woods | 1 | 0 |
+----+-----------+----------+---------+-------------+
Here Kevin is Acting for for John, and Michael is also a Programmer, so the result of my query should be:
+----+-------------------------+
| id | NAME |
+----+-------------------------+
| 1 | John Smith - Kevin Tull |
| 3 | Michael Woods |
| x | Other Programmers.. |
+----+-------------------------+

Answer 1

This untested query should give you the result you whant:
SELECT person1.id, CASE WHEN person1.actingas_id =0 then CONCAT(person1.firstname,' ', person1.lastname) else CONCAT(person1.firstname,' ', person1.lastname,' - ', person2.firstname,' ', person2.lastname) AS FULLNAME
FROM person person1 left join person person2 on person1.actingas_id=person2.id
join role on person1.role_id=role.id
WHERE role.name = 'Programmer'

Answer 2

Use UNION ALL to add those additional records:
SELECT person.id,
CONCAT(person.firstname,' ', person.lastname) AS FULLNAME
FROM person INNER JOIN role
ON person.role_id = role.id
WHERE role.name = 'Programmer'
AND NOT EXISTS(SELECT 1 FROM person p WHERE p.actingas_id = person.id)
UNION ALL
SELECT a.id,
CONCAT(a.firstname,' ', a.lastname, ' - ', person.firstname,' ', person.lastname) AS FULLNAME
FROM person INNER JOIN person a
ON a.acting_as = person.id
INNER JOIN role
ON person.role_id = role.id
WHERE role.name = 'Programmer' AND a.actingas_id <> 0
Also, avoid using old style comma-separated JOINs. Use INNER JOINs