ORA-00904 invalid identifier in oracle solution and commands

Oracle error ORA-00904: invalid identifier occurs when you try to reference a column or an alias in your SQL query that does not exist or is misspelled. Here are the common causes and solutions.

Common Causes:

Misspelled Column Name:

You may have misspelled a column name in your SQL query. Oracle is case-sensitive for column names that are enclosed in double quotes.

Solution:

Double-check your column names and ensure they match exactly (especially if they are enclosed in double quotes). For example:

SELECT first_name, last_name FROM employees;

Referencing a Non-Existent Column:

You might be trying to use a column that doesn’t exist in the table.

Solution:

Verify the column exists in the table using a query like:

DESC table_name;

Using an Alias in the WHERE or GROUP BY Clause:

You might have used an alias for a column in the SELECT clause, but SQL doesn’t allow you to reference an alias in the WHERE or GROUP BY clause.

Solution:

Instead of using the alias, use the full expression in the WHERE or GROUP BY clause. For example:

SELECT first_name || ' ' || last_name AS full_name

FROM employees

WHERE first_name || ' ' || last_name = 'John Doe';

Incorrect Use of Quotes:

When using double quotes around identifiers, Oracle treats them as case-sensitive, and the identifier must exactly match the case.

Solution:

Avoid using double quotes unless absolutely necessary, and if used, ensure the identifier matches the case:

SELECT "FirstName" FROM employees; -- "FirstName" is case-sensitive

Column is Missing from a Table in a Join:

When joining tables, ensure the columns you reference exist in the respective tables.

Solution:

Make sure you are referencing columns correctly in each table.

Example of Correcting an ORA-00904 Error:

Incorrect Query:

SELECT emp_id, first_name, last_name
FROM employees
WHERE firs_name = 'John';  -- Misspelled 'first_name'

Correct Query:

SELECT emp_id, first_name, last_name
FROM employees
WHERE first_name = 'John';  -- Correct column name