Ora-00904 string: invalid identifier

 ORA-00904 string: invalid identifier

    Oracle Database Error Code Messages ORA-00904 About Help Description;

String: invalid identifier.

    Problem Error Cause;

The column name entered is either missing or invalid.

    How to Solve; ORA-00904 string: invalid identifier?

    Fix Action for Solve Ora-00904;

    Enter a valid column name. A valid column name must begin with a letter, be less than or equal to 30 characters, and consist of only alphanumeric characters and the special characters $, _, and #. If it contains other characters, then it must be enclosed in double quotation marks. It may not be a reserved word.