Aggregates (set functions)

This section describes aggregates (also described as set functions in ANSI SQL and as column functions in some database literature).

Aggregates provide a means of evaluating an expression over a set of rows. Whereas the other built-in functions operate on a single expression, aggregates operate on a set of values and reduce them to a single scalar value. Built-in aggregates can calculate the minimum, maximum, sum, count, and average of an expression over a set of values as well as count rows.

In addition to the built-in aggregates, Derby allows you to create custom aggregate operators, called user-defined aggregates (UDAs). For information on creating and removing UDAs, see CREATE DERBY AGGREGATE statement and DROP DERBY AGGREGATE statement. See GRANT statement and REVOKE statement for information on usage privileges for UDAs.

For information on writing the Java classes that implement UDAs, see "Programming user-defined aggregates" in the Derby Developer's Guide.

The built-in aggregates can operate on expressions that evaluate to the data types shown in the following table.

Table 1. Permitted data types for built-in aggregates
Function Name Permitted Data Types
COUNT All types
MIN Data types that can be indexed
MAX Data types that can be indexed
AVG Numeric built-in data types
SUM Numeric built-in data types
Aggregates are permitted only in the following:
All expressions in selectItems in the selectExpression must be either aggregates or grouped columns (see GROUP BY clause). (The same is true if there is a HAVING clause without a GROUP BY clause.) This is because the ResultSet of a selectExpression must be either a scalar (single value) or a vector (multiple values), but not a mixture of both. (Aggregates evaluate to a scalar value, and the reference to a column can evaluate to a vector.) For example, the following query mixes scalar and vector values and thus is not valid:
-- not valid
SELECT MIN(flying_time), flight_id
FROM Flights
Aggregates are not allowed on outer references (correlations). This means that if a subquery contains an aggregate, that aggregate cannot evaluate an expression that includes a reference to a column in the outer query block. For example, the following query is not valid because SUM operates on a column from the outer query:
    (SELECT t2.x
    FROM t2
    WHERE t2.y = SUM(t1.c3)) 

A cursor declared on a ResultSet that includes an aggregate in the outer query block is not updatable.