[sql] How to check if a column exists before adding it to an existing table in PL/SQL?

How do I add a simple check before adding a column to a table for an oracle db? I've included the SQL that I'm using to add the column.

ALTER TABLE db.tablename 
  ADD columnname NVARCHAR2(30);

To check column exists

select column_name as found
from user_tab_cols
where table_name = '__TABLE_NAME__'
and column_name = '__COLUMN_NAME__'

Reference link

Normally, I'd suggest trying the ANSI-92 standard meta tables for something like this but I see now that Oracle doesn't support it.

-- this works against most any other database
SELECT
    * 
FROM 
    INFORMATION_SCHEMA.COLUMNS C 
    INNER JOIN 
        INFORMATION_SCHEMA.TABLES T 
        ON T.TABLE_NAME = C.TABLE_NAME 
WHERE 
    C.COLUMN_NAME = 'columnname'
    AND T.TABLE_NAME = 'tablename'

Instead, it looks like you need to do something like

-- Oracle specific table/column query
SELECT
    * 
FROM
    ALL_TAB_COLUMNS 
WHERE
    TABLE_NAME = 'tablename'
    AND COLUMN_NAME = 'columnname'

I do apologize in that I don't have an Oracle instance to verify the above. If it does not work, please let me know and I will delete this post.

Or, you can ignore the error:

declare
    column_exists exception;
    pragma exception_init (column_exists , -01430);
begin
    execute immediate 'ALTER TABLE db.tablename ADD columnname NVARCHAR2(30)';
    exception when column_exists then null;
end;
/