Question

I have an update query that should update a field with the most frequent values from another table
this is how i do it in postgreSQL

UPDATE TABLE1 T1 
SET COLUMN_B = (SELECT COLUMN_B
                         FROM 
                         (SELECT COLUMN_A,COLUMN_B, COUNT(1) AS FREQUENCY
                                 FROM TABLE2 T2
                                GROUP BY COLUMN_A,COLUMN_B
                                 ORDER BY COLUMN_A,FREQUENCY DESC) QUERY1
                          WHERE QUERY1.COLUMN_A= T1.COLUMN_A

                    GROUP BY COLUMN_A,COLUMN_B,FREQUENCY
                    ORDER BY FREQUENCY DESC LIMIT 1
                   )

this works fine in postgresql , i want to do the same query in Oracle using rownum =1 but I have several issues :

1. I can’t put an ORDER BY inside the select of an update
2. If i decide to put the order by in the nested select (QUERY1), the nested select doesn’t understand the reference to table T1 (T2.COLUMN_A = T1.COLUMN_A gives an error that T1.COLUMN_A invalid identifier)

How can i do this in oracle? what i want is that T1.COLUMN_B be filled with the most frequent common value of COLUMN_B FROM T2 for each COLUMN_A.

Thank you for any help or suggestion

Answer 1

This can be done in Oracle uses analytic functions; don’t know whether the same code would work in PostgreSQL.

Creating a target table:

SQL> create table t23
  2    as select deptno, cast (null as number(7,2)) max_sal from dept
  3  /

Table created.
SQL> 

Now, the update:

SQL> update t23
  2  set max_sal = ( select sal from
  3                   ( select deptno, sal
  4                     , row_number() over (partition by deptno order by sal desc) rn
  5                      from emp )
  6                 where rn = 1
  7                 and deptno = t23.deptno )
  8  /

6 rows updated.

SQL>

Here is the outcome:

SQL> select * from t23
  2  /

    DEPTNO    MAX_SAL
---------- ----------
        10       5000
        20       3000
        30       3750
        40
        50       4500
        60

6 rows selected.

SQL> 

Just to confirm the results…

SQL> select deptno, max(sal)
  2  from emp
  3  where sal is not null
  4  group by deptno
  5  order by deptno
  6  /

    DEPTNO   MAX(SAL)
---------- ----------
        10       5000
        20       3000
        30       3750
        50       4500

SQL>