Selecting rows based on one column change

Question

Hi All -
I need to select the rows from a table (this table has 4 columns and has a primary key and have many records for that primary key).
I need to select all the rows for this primary key for which only one column (in this case "flag" column) has been changing from its past value.
How can I do this?
for example, table data is:
ID, flag, Profile,date
pk, Y, Profile4,01/10/10
pk, N, Profile4, 01/08/10
pk,N, Profile3, 01/07/10
pk,Y, Profile3, 01/05/10
pk,Y, Profile2, 01/03/10
pk,N, Profile1, 01/02/10
The output of the query I am looking for should return the following i.e. only retrieve rows whose previous flag value has been changed:
ID, flag, Profile,date
pk, Y, Profile4,01/10/10
pk,N, Profile3, 01/07/10
pk,Y, Profile2, 01/03/10
pk,N, Profile1, 01/02/10
Please share your expertise.
Thanks,
Seenu

Answer 1

Hi, Seenu,
Centinul wrote:
It is always helpful to provide the following:
1. Oracle version (SELECT * FROM V$VERSION)
2. Sample data in the form of CREATE / INSERT statements.
3. Expected output
4. Explanation of expected output (A.K.A. "business logic")
5. Use \ tags for #2 and #3. See FAQ (Link on top right side) for details.
Seenu001 wrote:(1) Oracle Database 10g Enterprise Edition Release 10.2.0.3.0 - 64bi
PL/SQL Release 10.2.0.3.0 - Production
CORE 10.2.0.3.0 Production
TNS for Solaris: Version 10.2.0.3.0 - Production
NLSRTL Version 10.2.0.3.0 - Production Good!
(2) select 'pk' id,'Y' flag,'Profile4' profile,to_date('01/10/10','mm/dd/yy') dt from dual union all
select 'pk','N','Profile4',to_date('01/08/10','mm/dd/yy') from dual union all
select 'pk','N','Profile3',to_date('01/07/10','mm/dd/yy') from dual union all
select 'pk','Y','Profile3',to_date('01/05/10','mm/dd/yy') from dual union all
select 'pk','Y','Profile2',to_date('01/03/10','mm/dd/yy') from dual union all
select 'pk','N','Profile1',to_date('01/02/10','mm/dd/yy') from dualNot bad. It can be made into a CREATE TABLE AS statement without a lot of work, but if you're not willing to do that little bit or work, why should anyone else?     
(3) Expected output: I want all the rows from this table where the flags have been changed from its previous value.That's a vague description. Post the exat output, like Solomon did:
ID F PROFILE  DT
pk N Profile1 01/02/2010
pk Y Profile2 01/03/2010
pk N Profile3 01/07/2010
pk Y Profile4 01/10/2010
(4) I want all the rows from this table where the flags have been changed from its previous value.That's a repeat of the vague description. Explain how you get the crorrect results, with specific examples, like this:
"The row dated January 2 should be included, because it is the oldest row in the table, regardless of what its flag is.
The row dated January 3 should be included because ...
The row dated January 5 should *not* be included, because ..."
You don't have to explain every row, just every reason.
Solomon - I tried your query but the prev_flag is not being populated, it shows null value '-'NULL is not the same as '-'. Did you issue a command like
SET NULL "-"that would display a '-' instead of NULL?
Solomon's query works fine for me, as it did for him. Post your code, even if you just copied it from this site. (There may have been an editing problem.)
Frank - I tried your query, prev_flag is populated in this case but when I am using with the WITH clause, I am getting an error saying "unsupported column aliasing" , I guess WITH command doesnt allow use of column alias. The problem is that I forgot the keyword AS after the sub-query name.
WITH     got_prev_flag     AS
     SELECT     id, flag, profile, dt     -- DATE is not a good column name
     ,     LAG (flag, 1, '??') OVER (ORDER BY dt)  AS prev_flag
     FROM     t
--     WHERE     ...     -- any filtering goes here
SELECT     id, flag, profile, dt
FROM     got_prev_flag
WHERE     flag     != prev_flag
;If you don't post data in an executable form, then people can't test their ideas. If people can't test their ideas, trivial mistakes like this are very likely.
This flag column will have either
Y or N columns. Could you tell me what is the use of "??" value.By default, LAG returns NULL for the first row. If you want to include the first row in the results, then "WHERE     flag     != prev_flag" won't work when prev_flag is NULL. We could explicitly check for "prev_flag IS NULL", but I find it cleared to override the default, and have LAG return '??' on the first row. I guessed that '??' would make the condition "WHERE     flag     != prev_flag" work, no matter what value flag had. There's nothing magical about '??'; you can use any string that will never actually match flag.