Oracle/PLSQL: ORA-00979 Error

Error:

ORA-00979: not a GROUP BY expression

Cause:

You tried to execute a SELECT statement that included a GROUP BY function (ie: MIN, MAX, SUM, COUNT) and an expression in the SELECT list that was not in the GROUP BY clause.

Action:

The options to resolve this Oracle error are:

Rewrite the SELECT statement so that the column or expression listed in the SELECT list is also found in the GROUP BY clause.
Remove the GROUP BY function (ie: MIN, MAX, SUM, COUNT) from the SELECT statement.
Remove the expression from the SELECT list that was not in the GROUP BY clause.

For example, if you had tried to execute the following SELECT statement:

SELECT department, company, MIN(salary) as "Lowest salary"
FROM employees
GROUP BY department;

You would receive the following error message:

You could correct this by including company in the GROUP BY clause as follows:

SELECT department, company, MIN(salary) as "Lowest salary"
FROM employees
GROUP BY department, company;