MySQL - Problema de agrupamento de tabela dinâmica dinâmica

SELECT
  es.employee_id,
  CONCAT(e.first_name, " ", e.last_name) AS employee,
  MAX(IF (es.skill_id = 1, es.date_trained, null)) AS '1',
  MAX(IF (es.skill_id = 2, es.date_trained, null)) AS '2',
  MAX(IF (es.skill_id = 3, es.date_trained, null)) AS '3'
FROM
  employee_skills es
  LEFT JOIN employees e ON es.employee_id = e.id
GROUP BY
  es.employee_id 

| employee_id | employee   | 1          | 2          | 3          |
+-------------+------------+------------+------------+------------+
| 1001675     | Person Two | (null)     | 2016-07-02 | 2016-07-04 |
| 1006111     | Person One | 2016-07-01 | 2016-07-11 | (null)     |