Conditional Query for Employee Training Analysis Query - mysql

Question

I need to run training queries that return results for questions like the following: "Who has completed this training, but not that training?"
In the simplified table below, I would like to know which employee has completed training_id 1 (as indicated by a date in the completed_date field), but has not finished training_id 7.
+-------------+-------------+----------------+
| emp_id | training_id | completed_date |
+-------------+-------------+----------------+
| 1 | 1 | 2010-04-02 |
+-------------+-------------+----------------+
| 1 | 7 | Null |
+-------------+-------------+----------------+
| 2 | 1 | Null |
+-------------+-------------+----------------+
| 2 | 7 | Null |
+-------------+-------------+----------------+
Desired result would be emp_id 1 and we'd like to return his/her completed training and non-completed training per the query parameters:
+-------------+-------------+----------------+
| emp_id | training_id | completed_date |
+-------------+-------------+----------------+
| 1 | 1 | 2010-04-02 |
+-------------+-------------+----------------+
| 1 | 7 | Null |
+-------------+-------------+----------------+
I can't figure out how to do this with a regular query because it seems to need IF logic. Ex: Return rows where this one training is complete and return rows where this second training is not complete BUT ONLY if the first training is complete.
How do I express something like that in SQL?

Answer 1

You can use an EXISTS clause
SELECT t.*
FROM training t
# which employee has completed training_id 1
WHERE t.training_id = 1 and t.completed_date is not null
#but has not finished training_id 7.
AND NOT EXISTS (
SELECT * FROM training t2
where t2.emp_id=t.emp_id
and t2.training_id = 7
and t2.completed_date is not null)
If you wanted to test something more complex, like completed (4,5,6) but not (1,9), then you can use counts:
SELECT t.emp_id
FROM training t
WHERE t.training_id in (4,5,6) and t.completed_date is not null
group by t.emp_id
having count(distinct emp_id) = 3
AND NOT EXISTS (
SELECT * FROM training t2
where t2.emp_id=t.emp_id
and t2.training_id in (1,9)
and t2.completed_date is not null)
And finally if you need the full employee training record
SELECT e.*
FROM
(
SELECT t.emp_id
FROM training t
WHERE t.training_id in (4,5,6) and t.completed_date is not null
group by t.emp_id
having count(distinct emp_id) = 3
AND NOT EXISTS (
SELECT * FROM training t2
where t2.emp_id=t.emp_id
and t2.training_id in (1,9)
and t2.completed_date is not null)
) search
inner join training e on e.emp_id = search.emp_id
order by e.emp_id

Answer 2

select * from training where emp_id in
(
select distinct emp_id from training where completed_Date is not null
)

Answer 3

You could do a self-join on this table ("pretend it is two identical tables and join them"):
SELECT t1.emp_id, t1.training_id, t1.completed_date,
t2.training_id, t2.completed_date
FROM training AS t1 /* the aliases are needed for the self-join */
JOIN training AS t2
ON t1.emp_id = t2.emp_id
AND t2.training_id = 7 /* second training ID */
WHERE t1.training_id = 1 /* first training ID */
That should give you a result like this:
t1.emp_id | t1.training_id | t1.completed_date | t2.training_id | t2.completed_date
1 1 2010-04-02 7 NULL
2 1 NULL 7 NULL
You can then further limit the WHERE query, e.g. with
AND t1.completed_date IS NOT NULL
AND t2.completed_date IS NULL
which will give you the set that you want - training 1 completed, training 7 not.