Creating a "First" identifier in an SQL query - sql

Question

I need to be able to identify the first time a set of rows, a question set, occur in a view of a table. Not sure how to frame the question clearly so will try and set it out below.
Example data
+-----------+------------+--------+------+----------+--------+
| Reference | Date | PerRef | QSet | Question | Answer |
+-----------+------------+--------+------+----------+--------+
| 1 | 01/01/2015 | a | QS1 | Q1 | A1 |
| 1 | 01/01/2015 | a | QS1 | Q2 | A2 |
| 1 | 01/01/2015 | a | QS1 | Q3 | A3 |
| 2 | 02/01/2015 | a | QS1 | Q1 | A1 |
| 2 | 02/01/2015 | a | QS1 | Q2 | A2 |
| 2 | 02/01/2015 | a | QS1 | Q3 | A3 |
| 3 | 02/01/2015 | b | QS2 | Q1 | A1 |
| 3 | 02/01/2015 | b | QS2 | Q2 | A2 |
| 3 | 02/01/2015 | b | QS2 | Q3 | A3 |
| 4 | 02/01/2015 | b | QS3 | Q1 | A1 |
| 4 | 02/01/2015 | b | QS3 | Q2 | A2 |
| 4 | 02/01/2015 | b | QS3 | Q3 | A3 |
+-----------+------------+--------+------+----------+--------+
For this table I would want to generate a new column on a select query which would look like this:
+-----------+------------+---------+-------+----------+--------+----------+--------+
| Reference | Date | PerRef | QSet | Question | Answer | Earliest | Latest |
+-----------+------------+---------+-------+----------+--------+----------+--------+
| 1 | 01/01/2015 | a | QS1 | Q1 | A1 | 1 | 2 |
| 1 | 01/01/2015 | a | QS1 | Q2 | A2 | 1 | 2 |
| 1 | 01/01/2015 | a | QS1 | Q3 | A3 | 1 | 2 |
| 2 | 02/01/2015 | a | QS1 | Q1 | A1 | 2 | 1 |
| 2 | 02/01/2015 | a | QS1 | Q2 | A2 | 2 | 1 |
| 2 | 02/01/2015 | a | QS1 | Q3 | A3 | 2 | 1 |
| 3 | 02/01/2015 | b | QS2 | Q1 | A1 | 1 | 1 |
| 3 | 02/01/2015 | b | QS2 | Q2 | A2 | 1 | 1 |
| 3 | 02/01/2015 | b | QS2 | Q3 | A3 | 1 | 1 |
| 4 | 05/01/2015 | b | QS3 | Q1 | A1 | 1 | 1 |
| 4 | 05/01/2015 | b | QS3 | Q2 | A2 | 1 | 1 |
| 4 | 05/01/2015 | b | QS3 | Q3 | A3 | 1 | 1 |
+-----------+------------+---------+-------+----------+--------+----------+--------+
I have this in both Earliest and Latest as it then lets me pull back the first completed or the latest. Generally queries will be to bring back just one of one type of question set against a person, either the first completed or he latest. Having both these will of course enable the option to bring back the earliest OR latest and see the change between the two.
So to try and rephrase this - I need to create a counter based on the Reference, grouped by the PerRef and QSet, ordered by the date (either ascending or descending)
Of course if there is a better way please state as I may be making this overly complex - if there a way to select First or Last Reference grouped on PerRef and Qset?

Answer 1

I managed to find a solution using row_number():
SELECT t.*, orderT.Earliest
FROM #tmp_table t JOIN (SELECT Reference, PerRef, Qset, row_number() OVER (PARTITION BY PerRef, Qset order by Date ) as Earliest
FROM #tmp_table
GROUP BY Reference, Date, PerRef, Qset) orderT ON t.Reference = orderT.Reference
AND t.PerRef = orderT.PerRef
AND t.Qset = orderT.Qset
What it does is:
1. Compute the Earliest column for each Reference/PerRef/Qset combination
2. "Apply" it to each line using JOIN on Reference/PerRef/Qset
Check out row_number() documentation if you have trouble understanding the query:
https://msdn.microsoft.com/en-us/library/ms186734.aspx
Generating the Latestcolumn using a similar method shouldn't be a problem.