Grouping Rows with GROUP BY
GROUP BY clause enables a query to return summary information about groups of rows that share column values instead of returning each qualifying row. The complete syntax of
GROUP BY is:
GROUP BY col [COLLATE collation] [, col [COLLATE collation] ...]
For example, consider two cursor declarations. The first declaration returns the names of all employees each department, and arranges retrieval in ascending alphabetic order by department and employee name.
EXEC SQL DECLARE DEPT_EMP CURSOR FOR SELECT DEPARTMENT, LAST_NAME, FIRST_NAME FROM DEPARTMENT D, EMPLOYEE E WHERE D.DEPT_NO = E.DEPT_NO ORDER BY DEPARTMENT, LAST_NAME, FIRST_NAME;
In contrast, the next cursor illustrates the use of aggregate functions with
GROUP BY to return results known as group aggregates. It returns the average salary of all employees in each department. The
GROUP BY clause assures that average salaries are calculated and retrieved based on department names, while the
ORDER BY clause arranges retrieved rows alphabetically by department name.
EXEC SQL DECLARE AVG_DEPT_SAL CURSOR FOR SELECT DEPARTMENT, AVG(SALARY) FROM DEPARTMENT D, EMPLOYEE E WHERE D.DEPT_NO = E.DEPT_NO GROUP BY DEPARTMENT ORDER BY DEPARTMENT;
Collation Order in a GROUP BY Clause
VARCHAR columns are grouped in a
SELECT statement, it can be necessary to specify a collation order for the grouping, especially if columns used for grouping use different collation orders.
To specify the collation order to use for grouping columns in the
GROUP BY clause, include a
COLLATE clause after the column name. For example, in the following
GROUP BY clause, the collation order for two columns is specified:
. . . GROUP BY LNAME COLLATE FR_CA, FNAME COLLATE FR_CA;
For more information about collation order and a list of collation orders available in InterBase, see the Data Definition Guide.
Limitations of GROUP BY
GROUP BY, be aware of the following limitations:
- Each column name that appears in a
GROUP BYclause must also be specified in the
GROUP BYcannot specify a column whose values are derived from a mathematical, aggregate, or user-defined function.
GROUP BYcannot be used in
- Contain an
INTOclause (singleton selects).
- Use a subquery with a
FROMclause which references a view whose definition contains a
- Contain an
- For each
SELECTclause in a query, including subqueries, there can only be one
Enhancements to GROUP BY
InterBase 2017 Update 1 introduces enhancements to the GROUP BY syntax. For more information refer to: Enhancements to GROUP BY and ORDER BY.