Oracle SQL - comparing columns

Question

Using oracle SQL, in the data set below, I want to examine when column B = 'N', I want it to find the same ID in column A and compare what the rows in column C are for that ID. if they are the same then 'Y', if not then 'N' else null.

A   B   C       D
001 Y   Pizza   Pepperoni
002 Y   Pizza   Pepperoni
003 Y   Pizza   Pepperoni
003 N   Pizza   Sausage
004 Y   Pizza   Pepperoni
005 Y   Pizza   Pepperoni
005 N   Pizza   Sausage
005 N   Hamburger   Cheese

Ideally, I would run it to return the ID (column A) and the results of the case statement such that it looks like this...

A      B
001    (Null)
002    (Null)
003    (Null)
003    Y
004    (Null)
005    (Null)
005    Y
005    N

Can anyone provide me what code you would use to accomplish this?

Answer 1

As the other comments state, given your stated rules the second last row should have a value 'N' in B. The following script returns the correct result according to the stated rules, but doesn't match your sample output:

/* test data */ 
select '001' as a, 'Y' as b, 'Pizza' as c,       'Pepperoni' as d into testtable union all
select '002' as a, 'Y' as b, 'Pizza' as c,       'Pepperoni' as d union all
select '003' as a, 'Y' as b, 'Pizza' as c,       'Pepperoni' as d union all
select '003' as a, 'N' as b, 'Pizza' as c,       'Sausage' as d union all
select '004' as a, 'Y' as b, 'Pizza' as c,       'Pepperoni' as d union all
select '005' as a, 'Y' as b, 'Pizza' as c,       'Pepperoni' as d union all
select '005' as a, 'N' as b, 'Pizza' as c,       'Sausage' as d union all
select '005' as a, 'N' as b, 'Hamburger' as c,   'Cheese' as d
go

/* script */
select
   a
  ,case when b = 'Y' then null else
    case when c = lag(c, 1) over (partition by a order by c desc, d) then 'Y' else 'N' end
    end as b

from testtable
go


/* Results...*/

|-----+------|
| A   | B    |
|-----+------|
| 001 | NULL |
| 002 | NULL |
| 003 | NULL |
| 003 | Y    |
| 004 | NULL |
| 005 | NULL |
| 005 | Y    |
| 005 | N    |
|-----+------|