ORA-00979 not a group by expression

Question

You must put all columns of the SELECT in the GROUP BY or use functions on them which compress the results to a single value (like MIN, MAX or SUM).

A simple example to understand why this happens: Imagine you have a database like this:

FOO BAR
0   A
0   B

and you run SELECT * FROM table GROUP BY foo. This means the database must return a single row as result with the first column 0 to fulfill the GROUP BY but there are now two values of bar to chose from. Which result would you expect - A or B? Or should the database return more than one row, violating the contract of GROUP BY?

Include in the GROUP BY clause all SELECT expressions that are not group function arguments.

Too bad Oracle has limitations like these. Sure, the result for a column not in the GROUP BY would be random, but sometimes you want that. Silly Oracle, you can do this in MySQL/MSSQL.

BUT there is a work around for Oracle:

While the following line does not work

SELECT unique_id_col, COUNT(1) AS cnt FROM yourTable GROUP BY col_A;

You can trick Oracle with some 0's like the following, to keep your column in scope, but not group by it (assuming these are numbers, otherwise use CONCAT)

SELECT MAX(unique_id_col) AS unique_id_col, COUNT(1) AS cnt 
FROM yourTable GROUP BY col_A, (unique_id_col*0 + col_A);

If you do grouping by virtue of including GROUP BY clause, any expression in SELECT, which is not group function (or aggregate function or aggregated column) such as COUNT, AVG, MIN, MAX, SUM and so on (List of Aggregate functions) should be present in GROUP BY clause.

Example (correct way) (here employee_id is not group function (non-aggregated column), so it must appear in GROUP BY. By contrast, sum(salary) is a group function (aggregated column), so it is not required to appear in the GROUP BYclause.

   SELECT employee_id, sum(salary) 
   FROM employees
   GROUP BY employee_id; 

Example (wrong way) (here employee_id is not group function and it does not appear in GROUP BY clause, which will lead to the ORA-00979 Error .

   SELECT employee_id, sum(salary) 
   FROM employees;

To correct you need to do one of the following :

• Include all non-aggregated expressions listed in SELECT clause in the GROUP BY clause
• Remove group (aggregate) function from SELECT clause.