Invalid Column Name message in the subquery

Question

Hi All, I am receiving the invalid column name message in the subquery. Please help me if there is any work around or change in the query:
Sample query:
update table1 set table1.column1 =
(Select column1 from
(Select column1 from table2 where
table1.columnX = table2.columnX
order by table2.columnM desc,table2.ColumnN desc,
where rownum =1)
When i run the above query i am receiving the following error message:
ORA-00904:invalid column name at table1.columnx.
This column is already existing in table1 and looks like this table is not recoginzed inside of the subquery.
I am trying to update table1.column1 using the first value from table2 after ordering by table2.columnM desc, table2.columnN desc. (ColumnM and ColumnN are the Date columns which i need to perform the sorting). I am new to sql/Plsql. Thanks in Advance for help....

Answer 1

The problem is that you can not refer to columns from the outer table from inside a second subquery inside your subquery. Only one level of subquery can be used to transport the column name,.
A solution can be to use groups and aggregation functions instead of rownum = 1.
The following might work.
untested
update table1
set table1.column1 =
  (Select max(column1) keep (dense_rank first over table2.columnM desc,table2.ColumnN desc)
   from table2
   where table1.columnX = table2.columnX
   ) Another way can be to to make a UNCORRELATED subquery first and then filter on that uncorrelated query with the column name from the outer statement.
This time I use a analytic function.
untested
update table1 u
set u.column1 = (select v1.column1
   from
     (Select t2.column1, t2.columnX, row_number over (partition by t2.column1, t2.columnX order by t2.columnM desc,t2.ColumnN desc) as rn
      from table2
      ) v1
   where v1.columnX = u.columnX
   and v1.rn = 1
where /* where clause is missing! */