Understanding row_number() and using it in an analytic function

Question

Dear all;
I have been playing around with row_number and trying to understand how to use it and yet I still cant figure it out...
I have the following code below
create table Employee(
    ID                 VARCHAR2(4 BYTE)         NOT NULL,
   First_Name         VARCHAR2(10 BYTE),
   Last_Name          VARCHAR2(10 BYTE),
    Start_Date         DATE,
    End_Date           DATE,
     Salary             Number(8,2),
   City               VARCHAR2(10 BYTE),
    Description        VARCHAR2(15 BYTE)
insert into Employee(ID,  First_Name, Last_Name, Start_Date,                     End_Date,                       Salary,  City,       Description)
             values ('01','Jason',    'Martin',  to_date('19960725','YYYYMMDD'), to_date('20060725','YYYYMMDD'), 1234.56, 'Toronto',  'Programmer');
insert into Employee(ID,  First_Name, Last_Name, Start_Date,                     End_Date,                       Salary,  City,       Description)
              values('02','Alison',   'Mathews', to_date('19760321','YYYYMMDD'), to_date('19860221','YYYYMMDD'), 6661.78, 'Vancouver','Tester')
insert into Employee(ID,  First_Name, Last_Name, Start_Date,                     End_Date,                       Salary,  City,       Description)
             values('03','James',    'Smith',   to_date('19781212','YYYYMMDD'), to_date('19900315','YYYYMMDD'), 6544.78, 'Vancouver','Tester')
insert into Employee(ID,  First_Name, Last_Name, Start_Date,                     End_Date,                       Salary,  City,       Description)
              values('04','Celia',    'Rice',    to_date('19821024','YYYYMMDD'), to_date('19990421','YYYYMMDD'), 2344.78, 'Vancouver','Manager')
insert into Employee(ID,  First_Name, Last_Name, Start_Date,                     End_Date,                       Salary,  City,       Description)
              values('05','Robert',   'Black',   to_date('19840115','YYYYMMDD'), to_date('19980808','YYYYMMDD'), 2334.78, 'Vancouver','Tester')
insert into Employee(ID,  First_Name, Last_Name, Start_Date,                     End_Date,                       Salary, City,        Description)
              values('06','Linda',    'Green',   to_date('19870730','YYYYMMDD'), to_date('19960104','YYYYMMDD'), 4322.78,'New York',  'Tester')
insert into Employee(ID,  First_Name, Last_Name, Start_Date,                     End_Date,                       Salary, City,        Description)
              values('07','David',    'Larry',   to_date('19901231','YYYYMMDD'), to_date('19980212','YYYYMMDD'), 7897.78,'New York',  'Manager')
insert into Employee(ID,  First_Name, Last_Name, Start_Date,                     End_Date,                       Salary, City,        Description)
               values('08','James',    'Cat',     to_date('19960917','YYYYMMDD'), to_date('20020415','YYYYMMDD'), 1232.78,'Vancouver', 'Tester')I did a simple select statement
select * from Employee e
and it returns this below
ID   FIRST_NAME LAST_NAME  START_DAT END_DATE      SALARY CITY       DESCRIPTION
01   Jason      Martin     25-JUL-96 25-JUL-06    1234.56 Toronto    Programmer
02   Alison     Mathews    21-MAR-76 21-FEB-86    6661.78 Vancouver  Tester
03   James      Smith      12-DEC-78 15-MAR-90    6544.78 Vancouver  Tester
04   Celia      Rice       24-OCT-82 21-APR-99    2344.78 Vancouver  Manager
05   Robert     Black      15-JAN-84 08-AUG-98    2334.78 Vancouver  Tester
06   Linda      Green      30-JUL-87 04-JAN-96    4322.78 New York   Tester
07   David      Larry      31-DEC-90 12-FEB-98    7897.78 New York   Manager
08   James      Cat        17-SEP-96 15-APR-02    1232.78 Vancouver  TesterI wrote another select statement with row_number. see below
SELECT first_name, last_name, salary, city, description, id,
   ROW_NUMBER() OVER(PARTITION BY description ORDER BY city desc) "Test#"
   FROM employee
   and I get this result
First_name  last_name   Salary         City             Description         ID         Test#
Celina          Rice         2344.78      Vancouver    Manager             04          1
David          Larry         7897.78      New York    Manager             07          2
Jason          Martin       1234.56      Toronto      Programmer        01          1
Alison         Mathews    6661.78      Vancouver   Tester               02          1 
James         Cat           1232.78      Vancouver    Tester              08          2
Robert        Black         2334.78     Vancouver     Tester              05          3
James        Smith         6544.78     Vancouver     Tester              03          4
Linda         Green        4322.78      New York      Tester             06           5
I understand the partition by which means basically for each associated group a unique number wiill be assigned for that row, so in this case since tester is one group, manager is another group, and programmer is another group then tester gets its own unique number for each row, manager as well and etc.What is throwing me off is the order by and how this numbering are assigned. why is
1 assigned to Alison Mathews for the tester group and 2 assigned to James Cat and 3 assigned Robert Black
I apologize if this is a stupid question, i have tried reading about it online and looking at the oracle documentation but that still dont fully understand why.

Answer 1

user13328581 wrote:
understanding row_number() and using it in an analytic functionROW_NUMBER () IS an analytic fucntion. Are you trying to use the results of ROW_NUMBER in another analytic function? If so, you need a sub-query. Analuytic functions can't be nested within other analytic functions.
...I have the following code below
... I did a simple select statementThanks for posting all that! It's really helpful.
... and I get this result
First_name  last_name   Salary         City             Description         ID         Test#
Celina          Rice         2344.78      Vancouver    Manager             04          1
David          Larry         7897.78      New York    Manager             07          2
Jason          Martin       1234.56      Toronto      Programmer        01          1
Alison         Mathews    6661.78      Vancouver   Tester               02          1 
James         Cat           1232.78      Vancouver    Tester              08          2
Robert        Black         2334.78     Vancouver     Tester              05          3
James        Smith         6544.78     Vancouver     Tester              03          4
Linda         Green        4322.78      New York      Tester             06           5... What is throwing me off is the order by and how this numbering are assigned. why is
1 assigned to Alison Mathews for the tester group and 2 assigned to James Cat and 3 assigned Robert Black That's determined by the analytic ORDER BY clause. Yiou said "ORDER BY city desc", so a row where city='Vancouver' will get a lower namber than one where city='New York', since 'Vancouver' comes after 'New York' in alphabetic order.
If you have several rows that all have the same city, then you can be sure that ROW_NUMBER will assign them consecutive numbers, but it's arbitrary which one of them will be lowest and which highest. For example, you have 5 'Tester's: 4 from Vancouver and 1 from New York. There's no particular reason why the one with first_name='Alison' got assinge 1, and 'James' got #2. If you run the same query again, without changing the table at all, then 'Robert' might be #1. It's certain that the 4 Vancouver rows will be assigned numbers 1 through 4, but there's no way of telling which of those 4 rows will get which of those 4 numbers.
Similar to a query's ORDER BY clause, the analytic ORDER BY clause can have two or more expressions. The N-th one will only be considered if there was a tie for all (N-1) earlier ones. For example "ORDER BY city DESC, last_name, first_name" would mena 'Vancouver' comes before 'New York', but, if multiple rows all have city='Vancouver', last_name would determine the order: 'Black' would get a lower number than 'Cat'. If you had multiple rows with city='Vancouver' and last_name='Black', then the order would be determined by first_name.