SQL statement to find the last promotion date - mysql

Question

Some help will be greatly appreciated. I am trying to find the last date of most recent promotion of all employees from a dataset. A sample of the dataset is as below.
DATASET -
mysql> select * from emp;
+----+-------+-----------+------------+
| id | empid | title | startdate |
+----+-------+-----------+------------+
| 1 | 111 | Associate | 2017-11-01 |
| 2 | 222 | ED | 2017-11-01 |
| 3 | 111 | Associate | 2017-12-01 |
| 4 | 222 | MD | 2017-12-01 |
| 5 | 111 | VP | 2018-01-01 |
| 6 | 222 | MD | 2018-01-01 |
| 7 | 111 | VP | 2018-02-01 |
| 8 | 222 | MD | 2018-02-01 |
+----+-------+-----------+------------+
The output I want -
+-------+-----------+----------------+
| empid | title | min(startdate) |
+-------+-----------+----------------+
| 222 | MD | 2017-12-01 |
| 111 | VP | 2018-01-01 |
+-------+-----------+----------------+
I tried this SQL -
mysql> select empid, title, min(startdate)
from ( select * from emp ) x
group by title, empid
order by empid desc;
It outputs all groups, like this -
+-------+-----------+----------------+
| empid | title | min(startdate) |
+-------+-----------+----------------+
| 222 | MD | 2017-12-01 |
| 222 | ED | 2017-11-01 |
| 111 | Associate | 2017-11-01 |
| 111 | VP | 2018-01-01 |
+-------+-----------+----------------+

Answer 1

You can filter on the latest title per employee with a correlated subquery, and then aggregate:
select empid, title, min(startdate)
from emp e
where e.title = (
select e1.title
from emp e1
where e1.empid = e.empid
order by startdate desc
limit 1
)
group by empid, title
order by empid desc
Demo on DB Fiddle:
empid | title | min(startdate)
----: | :---- | :-------------
222 | MD | 2017-12-01
111 | VP | 2018-01-01

Answer 2

Take the empid and the max(startdate) ONLY, as a subquery and join it back to the main table:
SELECT *
FROM emp e
INNER JOIN
(SELECT empid, Max(startdate) recentpromo FROM emp GROUP BY empid) x
ON x.empid = e.empid AND x.recentpromo = e.startdate
If you're on mysql8 you could use ROW_NUMBER for this as an alternative. If an employee is promoted twice on the same day this will pick up both records, though I think that unlikely to occur
Edit, so we can extend this another level:
SELECT
em.empid,
em.title,
min(em.startdate) as
FROM
emp em
INNER JOIN
(
SELECT e.empid, e.title
FROM emp e
INNER JOIN
(SELECT empid, Max(startdate) recentpromo FROM emp GROUP BY empid) x
ON x.empid = e.empid AND x.recentpromo = e.startdate
) frpt --find recent promo title
ON
em.empid = frpt.empid AND
em.title = frpt.title
GROUP BY
em.empid, em.title