SQL Language Enhancements

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(1.5) Improved Aggregate Handling

Arno Brinkman

Originally, grouped sets could be grouped only on named columns. In Firebird 1.0, it became possible to group by a UDF expression. In 1.5, several further extensions to the handling of aggregate functions and the GROUP BY clause now allow groupings to be made by the degree of columns in the output specification (their 1-based "ordinal left-to-right position", as in the ORDER BY clause) or by a variety of expressions.

Caution

Not all expressions are currently allowed inside the GROUP BY list. For example, concatenation is not allowed.

Group By syntax

SELECT ... FROM .... [GROUP BY group_by_list]

group_by_list : group_by_item [, group_by_list];

group_by_item : column_name | degree (ordinal)

| udf

| group_by_function;

group_by_function : numeric_value_function | string_value_function

| case_expression

;

numeric_value_function : EXTRACT '(' timestamp_part FROM value ')';

string_value_function : SUBSTRING '(' value FROM pos_short_integer ')'

| SUBSTRING '(' value FROM pos_short_integer FOR nonneg_short_integer ')' | KW_UPPER '(' value ')'

;

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SQL Language Enhancements

Important

The group_by_item cannot be a reference to any aggregate-function (including any that are buried inside an expression) from the same context.

HAVING

The having clause only allows aggregate functions or valid expressions that are part of the GROUP BY clause. Previously it was allowed to use columns that were not part of the GROUP BY clause and to use non-valid expressions.

ORDER BY

When the context is an aggregate statement, the ORDER BY clause only allows valid expressions that are aggregate functions or expression parts of the GROUP BY clause.

Previously it was allowed to use non-valid expressions.

Aggregate functions inside subqueries

It is now possible to use an aggregate function or expression contained in the GROUP BY clause inside a subquery.

Examples

SELECT r.RDB$RELATION_NAME,

MAX(r.RDB$FIELD_POSITION), (SELECT

r2.RDB$FIELD_NAME FROM

RDB$RELATION_FIELDS r2 WHERE

r2.RDB$RELATION_NAME = r.RDB$RELATION_NAME and r2.RDB$FIELD_POSITION = MAX(r.RDB$FIELD_POSITION))

FROM RDB$RELATION_FIELDS r

GROUP BY 1

/* ************ */ SELECT

rf.RDB$RELATION_NAME AS "Relationname", (SELECT

r.RDB$RELATION_ID FROM

RDB$RELATIONS r WHERE

r.RDB$RELATION_NAME = rf.RDB$RELATION_NAME) AS "ID",

COUNT(*) AS "Fields" FROM

RDB$RELATION_FIELDS rf GROUP BY

rf.RDB$RELATION_NAME

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SQL Language Enhancements

Mixing aggregate functions from different contexts

Aggregate functions from different contexts can be used inside an expression.

Examples

SELECT r.RDB$RELATION_NAME,

MAX(i.RDB$STATISTICS) AS "Max1", (SELECT

COUNT(*) || ' - ' || MAX(i.RDB$STATISTICS) FROM RDB$RELATION_FIELDS rf

WHERE

rf.RDB$RELATION_NAME = r.RDB$RELATION_NAME) AS "Max2"

FROM RDB$RELATIONS r

JOIN RDB$INDICES i on (i.RDB$RELATION_NAME = r.RDB$RELATION_NAME) GROUP BY

r.RDB$RELATION_NAME HAVING

MIN(i.RDB$STATISTICS) <> MAX(i.RDB$STATISTICS)

Note! This query gives results in FB1.0, but they are WRONG!

Subqueries are supported inside an aggregate function

Using a singleton SELECT expression inside an aggregate function is supported.

Example

SELECT r.RDB$RELATION_NAME, SUM((SELECT

COUNT(*) FROM

RDB$RELATION_FIELDS rf WHERE

rf.RDB$RELATION_NAME = r.RDB$RELATION_NAME))

FROM RDB$RELATIONS r

JOIN RDB$INDICES i

on (i.RDB$RELATION_NAME = r.RDB$RELATION_NAME) GROUP BY

r.RDB$RELATION_NAME

Nested aggregate functions

Using an aggregate function inside another aggregate function is possible if the inner aggregate function is from a lower context (see example above).

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