Firebird 2.1 Language Reference Update

GROUP BY
DML statements
Description: GROUP BY merges rows that have the same combination of values and/or NULLs in the item list
into a single row. Any aggregate functions in the select list are applied to each group individually instead of
to the dataset as a whole.
Syntax:
SELECT ... FROM ...
GROUP BY [, ...]
...
::= column-name [COLLATE collation-name]
| column-alias
| column-position
| expression
• Only non-negative integer literals will be interpreted as column positions. If they are outside the
range from 1 to the number of columns, an error is raised. Integer values resulting from expressions
or parameter substitutions are simply invariables and will be used as such in the grouping. They
will have no effect though, as their value is the same for each row.
• A GROUP BY item cannot be a reference to an aggregate function (including one that is buried
inside an expression) from the same context.
• The select list may not contain expressions that can have different values within a group. To avoid
this, the rule of thumb is to include each non-aggregate item from the select list in the GROUP BY
list (whether by copying, alias or position).
Note: If you group by a column position, the expression at that position is copied internally from the select list.
If it concerns a subquery, that subquery will be executed at least twice.
Grouping by alias, position and expressions
Changed in: 1.0, 1.5, 2.0
Description: In addition to column names, Firebird 2 allows column aliases, column positions and arbitrary
valid expressions as GROUP BY items.
Examples:
These three queries all achieve the same result:
select strlen(lastname) as len_name, count(*)
from people
group by len_name
select strlen(lastname) as len_name, count(*)
from people
group by 1
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