# GROUP BY clause

A GROUP BY clause, part of a selectExpression, groups a result into subsets that have matching values for one or more columns.

In each group, no two rows have the same value for the grouping column or columns. NULLs are considered equivalent for grouping purposes. See selectExpression for more information.

You typically use a GROUP BY clause in conjunction with an aggregate expression.

Using the ROLLUP syntax, you can specify that multiple levels of grouping should be computed at once.

## Syntax

    GROUP BY
    {
        columnName [ , columnName ]* |
        ROLLUP ( columnName [ , columnName ]* )
    }

The columnName must be a column from the current scope of the query; there can be no columns from a query block outside the current scope. For example, if a GROUP BY clause is in a subquery, it cannot refer to columns in the outer query.

The selectItems in the selectExpression with a GROUP BY clause must contain only aggregates or grouping columns.

## Examples

    -- find the average flying_times of flights grouped by
    -- airport
    SELECT AVG (flying_time), orig_airport
    FROM Flights
    GROUP BY orig_airport

    SELECT MAX(city_name), region
    FROM Cities, Countries
    WHERE Cities.country_ISO_code = Countries.country_ISO_code
    GROUP BY region

    -- group by an a smallint
    SELECT ID, AVG(SALARY)
    FROM SAMP.STAFF
    GROUP BY ID

    -- Get the AVGSALARY and EMPCOUNT columns, and the DEPTNO column using the AS clause
    -- And group by the WORKDEPT column using the correlation name OTHERS
    SELECT OTHERS.WORKDEPT AS DEPTNO,
    AVG(OTHERS.SALARY) AS AVGSALARY,
    COUNT(*) AS EMPCOUNT
    FROM SAMP.EMPLOYEE OTHERS
    GROUP BY OTHERS.WORKDEPT

    -- Compute sub-totals of Sales_History data, grouping it by Region, by
    -- (Region, State), and by (Region, State, Product), as well as computing
    -- an overall total of the sales for all Regions, States, and Products:
    SELECT Region, State, Product, SUM(Sales) Total_Sales
    FROM Sales_History
    GROUP BY ROLLUP(Region, State, Product)

Parent topic: SQL clauses