# Group

Relation-valued attribute

## Signature

group(operand: Relation, attributes: AttrList, as: AttrName) -> Relation

## Examples

group(suppliers, [:sid, :name, :status], :suppliers)

group(suppliers, [:city], :suppliers, allbut: true)

## Description

Summarizes `operand` by all but the specified `attributes` and groups the
latter under a relation-value attribute `as`.

This operator could be formally defined as the following shortcut:

def group(operand, attributes, as)
  extend(
    allbut(operand, attributes),
    as: ->(t){ project(matching(operand, Relation(t)), attributes) })
end
group(suppliers, [:sid, :name, :status], :suppliers)

This operators supports an ALL BUT variant, through the `allbut` option.
When set to true, the operator keeps specified attributes and groups all
remaining ones as a relation-valued attribute.

## Implementation notes

This operator does not compile to SQL so far. Contributions are welcome
to provide it with a SQL compilation for SQL DBMSs that support this kind
of feature (e.g. PostgreSQL with JSON data type)