Oracle Virtual Columns – Can’t use plain column or duplicate expressions

I had a scenario today where I was loading a table and a particular column is known by multiple names in different source systems and thus to different people. In order to make everyone happy on this occasion, I wondered if I could create a normal column for one of the multiple names and then use virtual columns pointing at the normal column, for the other names.

I’m aware there are several ways of skinning this cat and that virtual columns was probably not the best choice in the first place, but I was just playing with an idea and it didn’t quite end up where I thought…so it was interesting in that respect.

The test code:

CREATE TABLE test
  (
     column1    INTEGER NOT NULL 
    ,virtual_column1 INTEGER AS ( column1 ) VIRTUAL NOT NULL 
  )
/
CREATE TABLE test
  (
     column1    INTEGER NOT NULL 
    ,virtual_column1 INTEGER AS ( column1 + 0 ) VIRTUAL NOT NULL 
  )
/
DROP TABLE test PURGE
/
CREATE TABLE test
  (
     column1    INTEGER NOT NULL 
    ,virtual_column1 INTEGER AS ( column1 + 0 ) VIRTUAL NOT NULL 
    ,virtual_column2 INTEGER AS ( column1 + 0 ) VIRTUAL NOT NULL 
  )
/
CREATE TABLE test
  (
     column1    INTEGER NOT NULL 
    ,virtual_column1 INTEGER AS ( column1 + 0 ) VIRTUAL NOT NULL 
    ,virtual_column2 INTEGER AS ( column1 + 1 - 1 ) VIRTUAL NOT NULL 
  )
/
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/

And the results:

    ,virtual_column1 INTEGER AS ( column1 ) VIRTUAL NOT NULL
                                  *
ERROR at line 4:
ORA-54016: Invalid column expression was specified



Table created.


Table dropped.

    ,virtual_column2 INTEGER AS ( column1 + 0 ) VIRTUAL NOT NULL
     *
ERROR at line 5:
ORA-54015: Duplicate column expression was specified



Table created.


Table dropped.

I expected it to just work, but I clearly ran in to two problems which scuppered my idea. Firstly, the virtual column can’t simply refer to a normal column without any changes to it, otherwise it fails with ORA-54016. The error isn’t particularly helpful, but eventually I worked out that it was because the column is simply a mapping to a non virtual column. Working around that by adding zero to the numeric column gets it to work, but it’s an ugly hack.

In my scenario there are three different names for this column, depending on the users involved, which then leads on to the next issue, which is that I’d then need two virtual columns pointing at the same source column. Unfortunately if I use the same hack twice, it fails with ORA-54015, because you can’t have two virtual columns with exactly the same expression! A slight variant to the hack and it works, but it’s getting uglier and uglier!

Time to seek out plan B!