Oracle 11g Release 2

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create table Employees(employe, ID) AS
SELECT 'Person A',  1 FROM DUAL UNION ALL
SELECT 'Person B',  2 FROM DUAL UNION ALL
SELECT 'Person C',  3 FROM DUAL
3 rows affected

SELECT * FROM EMPLOYEES


EMPLOYE
ID


Person A
1

Person B
2

Person C
3


CREATE TABLE Projects(project, employe_id) AS
SELECT 'Project X',  ',2,' FROM DUAL UNION ALL
SELECT 'Project Y',  '' FROM DUAL UNION ALL
SELECT 'Project Z',  ',1,3,' FROM DUAL
3 rows affected

SELECT * FROM PROJECTS


PROJECT
EMPLOYE_ID


Project X
,2,

Project Y
null

Project Z
,1,3,


SELECT PROJECT, NVL(REGEXP_COUNT(EMPLOYE_ID, '[^,]'), 0) AS ID_COUNT
  FROM PROJECTS


PROJECT
ID_COUNT


Project X
1

Project Y
0

Project Z
2


WITH cteId_counts_by_project AS (SELECT PROJECT,
                                        NVL(REGEXP_COUNT(EMPLOYE_ID, '[^,]'), 0) AS ID_COUNT
                                   FROM PROJECTS),
     cteMax_id_count AS (SELECT MAX(ID_COUNT) AS MAX_ID_COUNT
                           FROM cteId_counts_by_project),
     cteProject_employee_ids AS (SELECT PROJECT,
                                        EMPLOYE_ID,
                                        REGEXP_SUBSTR(EMPLOYE_ID, '[^,]',1, LEVEL) AS ID
                                   FROM PROJECTS
                                   CROSS JOIN cteMax_id_count m
                                   CONNECT BY LEVEL <= m.MAX_ID_COUNT),
     cteProject_emps AS (SELECT DISTINCT PROJECT, ID
                           FROM cteProject_employee_ids
                           WHERE ID IS NOT NULL),
     cteProject_empnames AS (SELECT pe.PROJECT, pe.ID, e.EMPLOYE
                               FROM cteProject_emps pe
                               LEFT OUTER JOIN EMPLOYEES e
                                 ON e.ID = pe.ID
                               ORDER BY pe.PROJECT, e.EMPLOYE)
SELECT p.PROJECT,
       LISTAGG(pe.EMPLOYE, ',') WITHIN GROUP (ORDER BY pe.EMPLOYE) AS EMPLOYEE_LIST
  FROM PROJECTS p
  LEFT OUTER JOIN cteProject_empnames pe
    ON pe.PROJECT = p.PROJECT
  GROUP BY p.PROJECT
  ORDER BY p.PROJECT


PROJECT
EMPLOYEE_LIST


Project X
Person B

Project Y
null

Project Z
Person A,Person C