Selecting Distinct field and row num just to display an id number gives duplicated data

Question

I have a table application and it has 10 columns. category is one column and this column has duplicated values. To get distinct values I have a query

SELECT distinc

Answer 1

You need to use the subquery as follows:

select CategoryName , rownum as categoryId from
(SELECT distinct(CATEGORY) as CategoryName FROM APPLICATION where applicationId=?)

You can also use the analytical function as follows:

SELECT distinct(CATEGORY) as CategoryName, 
       rank() over (order by CATEGORY) as categoryId 
  FROM APPLICATION where applicationId=?

Example:

I have the following data:

SQL> SELECT WRITER_ID, TWEET FROM TWEET;

 WRITER_ID TWEET
---------- -----
         1 T1
         1 T2

SQL>

See the output of both of the above query:

SQL> SELECT WRITER_ID, ROWNUM AS GENERATED_NUMBER FROM
  2  (SELECT DISTINCT WRITER_ID AS WRITER_ID FROM TWEET);

 WRITER_ID GENERATED_NUMBER
---------- ----------------
         1                1

SQL> SELECT DISTINCT ( WRITER_ID ),
  2                  RANK() OVER(ORDER BY WRITER_ID) AS GENERATED_NUMBER
  3    FROM TWEET;

 WRITER_ID GENERATED_NUMBER
---------- ----------------
         1                1

Now, Let me change my data

SQL> UPDATE TWEET
  2     SET
  3  WRITER_ID = 2
  4   WHERE ID = 101;

1 row updated.

Table data is changed:

SQL> SELECT WRITER_ID, TWEET FROM TWEET;

 WRITER_ID TWEET
---------- -----
         1 T1
         2 T2

SQL>

Lets see what is the result of the above queries now:

SQL> SELECT WRITER_ID, ROWNUM AS GENERATED_NUMBER FROM
  2  (SELECT DISTINCT WRITER_ID AS WRITER_ID FROM TWEET);

 WRITER_ID GENERATED_NUMBER
---------- ----------------
         1                1
         2                2

SQL> SELECT DISTINCT ( WRITER_ID ),
  2                  RANK() OVER(ORDER BY WRITER_ID) AS GENERATED_NUMBER
  3    FROM TWEET;

 WRITER_ID GENERATED_NUMBER
---------- ----------------
         2                2
         1                1