ORA-00904

ORA-00904: invalid identifier

Cause: The column name entered is either missing or invalid.
Action: 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.
ORA-00904 is a very simple issue. ORA-00904 may occur when we try to create or alter a table with invalid column name. It also may occur when we try to reference a non existing column in a select / insert / update / delete statement.

Examples which may lead to ORA-00904 are following:

SQL> CREATE TABLE TEST
  2  (
  3     ID      NUMBER,
  4     NAME    VARCHAR2(200),
  5     COMMENT VARCHAR2(4000)
  6  );
        COMMENT VARCHAR2(4000)
        *
ERROR at line 5:
ORA-00904: invalid identifier

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SQL> select empid from scott.emp;
select empid from scott.emp
       *
ERROR at line 1:
ORA-00904: "EMPID": invalid identifier

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SQL> update scott.emp set salary=1000 where empno = 3625;
update scott.emp set salary=1000 where empno = 3625
                     *
ERROR at line 1:
ORA-00904: "SALARY": invalid identifier

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SQL> delete scott.emp where empid = 3625;
delete scott.emp where empid = 3625
                       *
ERROR at line 1:
ORA-00904: "EMPID": invalid identifier

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SQL> insert into scott.emp (empno, empname, sal) 
  2  values(3625, 'Amit', 10000);
insert into scott.emp (empno, empname, sal)
                              *
ERROR at line 1:
ORA-00904: "EMPNAME": invalid identifier

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SQL> select emp.ename from scott.emp e;
select emp.ename from scott.emp e
       *
ERROR at line 1:
ORA-00904: "EMP"."ENAME": invalid identifier

ORA-00904 can simply be avoided by using the valid column name in create or alter statement. Also for DML statements ORA-00904 can be avoided by making a valid reference to the column name or the alias.

A valid column name must follow following criteria
- The column name must begin with a letter.
- The column name can not be of more than 30 characters.
- The column name must be made up of alphanumeric characters
- The column name may contain following special characters: $, _, and #.
- If the column name uses any other characters, it must be enclosed in double quotation marks.
- The column name can not be a reserved word.