SQL - suppressing duplicate *adjacent* records - sql

Question

I need to run a Select statement (DB2 SQL) that does not pull adjacent row duplicates based on a certain field. In specific, I am trying to find out when data changes, which is made difficult because it might change back to its original value.
That is to say, I have a table that vaguely resembles the below, sorted by Letter and then by Date:
A, 5, 2009-01-01
A, 12, 2009-02-01
A, 12, 2009-03-01
A, 12, 2009-04-01
A, 9, 2009-05-01
A, 9, 2009-06-01
A, 5, 2009-07-01
And I want to get the results:
A, 5, 2009-01-01
A, 12, 2009-02-01
A, 9, 2009-05-01
A, 5, 2009-07-01
discarding adjacent duplicates but keeping the last row (despite it having the same number as the first row). The obvious:
Select Letter, Number, Min(Update_Date) from Table group by Letter, Number
does not work -- it doesn't include the last row.
Edit: As there seems to have been some confusion, I have clarified the month column into a date column. It was meant as a human-parseable short form, not as actual valid data.
Edit: The last row is not important BECAUSE it is the last row, but because it has a "new value" that is also an "old value". Grouping by NUMBER would wrap it in with the first row; it needs to remain a separate entity.

Answer 1

Depending on which DB2 you're on, there are analytic functions which can make this problem easy to solve. An example in Oracle is below, but the select syntax appears to be pretty similar.
create table t1 (c1 char, c2 number, c3 date);
insert into t1 VALUES ('A', 5, DATE '2009-01-01');
insert into t1 VALUES ('A', 12, DATE '2009-02-01');
insert into t1 VALUES ('A', 12, DATE '2009-03-01');
insert into t1 VALUES ('A', 12, DATE '2009-04-01');
insert into t1 VALUES ('A', 9, DATE '2009-05-01');
insert into t1 VALUES ('A', 9, DATE '2009-06-01');
insert into t1 VALUES ('A', 5, DATE '2009-07-01');
SQL> l
1 SELECT C1, C2, C3
2 FROM (SELECT C1, C2, C3,
3 LAG(C2) OVER (PARTITION BY C1 ORDER BY C3) AS PRIOR_C2,
4 LEAD(C2) OVER (PARTITION BY C1 ORDER BY C3) AS NEXT_C2
5 FROM T1
6 )
7 WHERE C2 <> PRIOR_C2
8 OR PRIOR_C2 IS NULL -- to pick up the first value
9 ORDER BY C1, C3
SQL> /
C C2 C3
- ---------- -------------------
A 5 2009-01-01 00:00:00
A 12 2009-02-01 00:00:00
A 9 2009-05-01 00:00:00
A 5 2009-07-01 00:00:00

Answer 2

This is not possible with set based commands (i.e. group by and such).
You may be able to do this by using cursors.
Personally, I would get the data into my client application and do the filtering there.

Answer 3

The first thing you'd have to do is identify the sequence within which you wish to view/consider the the data. Values of "Jan, Feb, Mar" don't help, because the data's not in alphabetical order. And what happens when you flip from Dec to Jan? Step 1: identify a sequence that uniquely defines each row with regards to your problem.
Next, you have to be able to compare item #x with item #x-1, to see if it has changed. If changed, include; if not changed, exclude. Trivial when using procedural code loops (cursors in SQL), but would you want to use those? They tend not to perform too well.
One SQL-based way to do this is to join the table on itself, with the join clause being "MyTable.SequenceVal = MyTable.SequenceVal - 1". Throw in a comparison, make sure you don't toss the very first row of the set (where there is no x-1), and you're done. Note that performance may suck if the "SequenceVal" is not indexed.

Answer 4

Using an "EXCEPT" clause is one way to do it. See below for the solution. I've included all of my test steps here. First, I created a session table (this will go away after I disconnect from my database).
CREATE TABLE session.sample (
letter CHAR(1),
number INT,
update_date DATE
);
Then I imported your sample data:
IMPORT FROM sample.csv OF DEL INSERT INTO session.sample;
Verified that your sample information is in the database:
SELECT * FROM session.sample;
LETTER NUMBER UPDATE_DATE
------ ----------- -----------
A 5 01/01/2009
A 12 02/01/2009
A 12 03/01/2009
A 12 04/01/2009
A 9 05/01/2009
A 9 06/01/2009
A 5 07/01/2009
7 record(s) selected.
I wrote this with an EXCEPT clause, and used the "WITH" to try to make it clearer. Basically, I'm trying to select all rows that have a previous date entry. Then, I exclude all of those rows from a select on the whole table.
WITH rows_with_previous AS (
SELECT s.*
FROM session.sample s
JOIN session.sample s2
ON s.letter = s2.letter
AND s.number = s2.number
AND s.update_date = s2.update_date - 1 MONTH
)
SELECT *
FROM session.sample
EXCEPT ALL
SELECT *
FROM rows_with_previous;
Here is the result:
LETTER NUMBER UPDATE_DATE
------ ----------- -----------
A 5 01/01/2009
A 12 04/01/2009
A 9 06/01/2009
A 5 07/01/2009
4 record(s) selected.