Select most occurrences of a varchar2 in Oracle
Here is one way to do it using window functions:
select ProductNumber, Description
from (
  select ProductNumber, Description
       , row_number() over (partition by productnumber order by cnt1, cnt2 desc) as rn
  from (
    select ProductNumber, Description
         , count(1) over (partition by ProductNumber) as cnt1
         , count(1) over (partition by ProductNumber, Description) as cnt2
    from inventory
  ) t
) u 
where rn = 1;

EDIT: Added plans for a slightly bigger set of data, Fiddle
Verace query
-----------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                  | Name      | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |           |      1 |        |    676 |00:00:00.01 |     122 |       |       |          |
|   1 |  SORT UNIQUE               |           |      1 |  17576 |    676 |00:00:00.01 |     122 | 46080 | 46080 |40960  (0)|
|*  2 |   HASH JOIN SEMI           |           |      1 |  17576 |  17576 |00:00:00.01 |     122 |  2178K|  2050K| 2666K (0)|
|   3 |    TABLE ACCESS FULL       | INVENTORY |      1 |  17576 |  17576 |00:00:00.01 |      61 |       |       |          |
|*  4 |    VIEW                    |           |      1 |  17576 |    676 |00:00:00.01 |      61 |       |       |          |
|*  5 |     WINDOW SORT PUSHED RANK|           |      1 |  17576 |    676 |00:00:00.01 |      61 | 48128 | 48128 |43008  (0)|
|   6 |      HASH GROUP BY         |           |      1 |  17576 |    676 |00:00:00.01 |      61 |  1214K|  1214K| 1854K (0)|
|   7 |       TABLE ACCESS FULL    | INVENTORY |      1 |  17576 |  17576 |00:00:00.01 |      61 |       |       |          |
-----------------------------------------------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   2 - access(&quot;T1&quot;.&quot;DESCRIPTION&quot;=&quot;I2&quot;.&quot;DESCRIPTION&quot;)
   4 - filter(&quot;T1&quot;.&quot;RN&quot;=1)
   5 - filter(ROW_NUMBER() OVER ( PARTITION BY &quot;I1&quot;.&quot;PRODUCTNUMBER&quot; ORDER BY COUNT(*) DESC )&lt;=1)

H. Pauwelyn's modified query
------------------------------------------------------------------------------------------------------------------------
| Id  | Operation             | Name      | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |           |      1 |        |    676 |00:00:00.01 |     122 |       |       |          |
|   1 |  HASH UNIQUE          |           |      1 |    176 |    676 |00:00:00.01 |     122 |  2170K|  2170K| 1427K (0)|
|*  2 |   HASH JOIN RIGHT SEMI|           |      1 |    176 |    676 |00:00:00.01 |     122 |  2546K|  2546K|  654K (0)|
|   3 |    VIEW               |           |      1 |  17576 |    676 |00:00:00.01 |      61 |       |       |          |
|   4 |     HASH GROUP BY     |           |      1 |  17576 |    676 |00:00:00.01 |      61 |  1520K|  1520K| 1882K (0)|
|   5 |      TABLE ACCESS FULL| INVENTORY |      1 |  17576 |  17576 |00:00:00.01 |      61 |       |       |          |
|   6 |    VIEW               |           |      1 |  17576 |    676 |00:00:00.01 |      61 |       |       |          |
|   7 |     HASH GROUP BY     |           |      1 |  17576 |    676 |00:00:00.01 |      61 |  1520K|  1520K| 1882K (0)|
|   8 |      TABLE ACCESS FULL| INVENTORY |      1 |  17576 |  17576 |00:00:00.01 |      61 |       |       |          |
------------------------------------------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   2 - access(&quot;CNT1&quot;.&quot;TOTAL&quot;=&quot;CNT2&quot;.&quot;MAXTOTAL&quot;)

Lennart's query
----------------------------------------------------------------------------------------------------------------------
| Id  | Operation                | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |      |      1 |        |    676 |00:00:00.03 |     208 |       |       |          |
|*  1 |  VIEW                    |      |      1 |  17576 |    676 |00:00:00.03 |     208 |       |       |          |
|*  2 |   WINDOW SORT PUSHED RANK|      |      1 |  17576 |    676 |00:00:00.03 |     208 |  1116K|   557K|  991K (0)|
|   3 |    VIEW                  |      |      1 |  17576 |  17576 |00:00:00.02 |     208 |       |       |          |
|   4 |     WINDOW BUFFER        |      |      1 |  17576 |  17576 |00:00:00.02 |     208 |  1116K|   557K|  991K (0)|
|   5 |      INDEX FULL SCAN     | IX1  |      1 |  17576 |  17576 |00:00:00.01 |     208 |       |       |          |
----------------------------------------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   1 - filter(&quot;RN&quot;=1)
   2 - filter(ROW_NUMBER() OVER ( PARTITION BY &quot;PRODUCTNUMBER&quot; ORDER BY &quot;CNT1&quot;,INTERNAL_FUNCTION(&quot;CNT2&quot;) DESC 
              )&lt;=1)