ORA-00979: not a GROUP BY expression

ORA-00979 error in Oracle usually occurs when you try to execute a SQL statement that includes a GROUP BY clause, but one or more non-aggregated columns in the SELECT clause are not included in the GROUP BY clause.

Here's an example that demonstrates the issue:

SELECT dept_name, employee_name, AVG(salary)
FROM employees
GROUP BY dept_name;

This query calculates the average salary for each department in the employees table, but it includes the non-aggregated employee_name column in the SELECT clause without including it in the GROUP BY clause. This will result in the ORA-00979 error, because employee_name is not included in the GROUP BY clause and it's not part of an aggregate function.

To fix this error, you need to include all non-aggregated columns in the SELECT clause in the GROUP BY clause as well. Here's the corrected query:

SELECT dept_name, employee_name, AVG(salary)
FROM employees
GROUP BY dept_name, employee_name;

In this query, we include the employee_name column in the GROUP BY clause along with the dept_name column. This ensures that all non-aggregated columns in the SELECT clause are included in the GROUP BY clause, and the AVG function can be calculated for each combination of dept_name and employee_name.