group

Ruby

rel.group([:a, :b, ...])
rel.group([:a, :b, ...], :my_group)
rel.group([:a, :b, ...], :my_group, array: true)

TypeScript

rel.group(['a', 'b', ...], 'my_group')

Problem

Place tuples in groups based on common values for some attributes.

Example: Which are all our stores listed by city, with full details on each store?

Description

group splits the attributes of the input relation into two sets: the attributes given in the first argument, and all the remaining ones, called the grouping attributes. Below, we refer to these disjoint sets as grouped-attrs and grouping-attrs respectively.

group creates groups of grouped-attrs-tuples from the input, and pairs those groups with the corresponding grouping-attrs. We say that we group by the grouping-attrs, or, equivalently, that we group over the grouped-attrs.

To be more specific: the output consists of the unique tuples obtained by projecting the grouping-attrs from the input, and extending each such tuple with a nested relation with the grouped-attrs, such that for each combination of values in the input, a corresponding tuple is found in each nested relation.

Ruby

Optionally, as, the name of the attribute containing the nested relations, can be given. The default is :group.

If the option array: true is passed, the nested relations will be turned into arrays.

TypeScript

The second argument specifies the name of the attribute containing the nested relations.

Requirements

The specified attributes must be part of the input relation’s heading.

Examples

Consult the Overview page for the data model used in these examples.

Ruby

suppliers.group([:sid, :name, :status], :my_group).to_a

=>
[{:city=>"London",
  :my_group=>
   (in_memory #<Set: {{:sid=>"S1", :name=>"Smith", :status=>20}, {:sid=>"S4", :name=>"Clark", :status=>20}}>)},
 {:city=>"Paris",
  :my_group=>
   (in_memory #<Set: {{:sid=>"S2", :name=>"Jones", :status=>10}, {:sid=>"S3", :name=>"Blake", :status=>30}}>)},
 {:city=>"Athens", :my_group=>(in_memory #<Set: {{:sid=>"S5", :name=>"Adams", :status=>30}}>)}]

TypeScript

suppliers.group(['sid', 'name', 'status'], 'my_group').toArray()

// =>
// [{ city: "London",
//    my_group: Relation([{ sid: "S1", name: "Smith", status: 20 },
//                        { sid: "S4", name: "Clark", status: 20 }]) },
//  { city: "Paris",
//    my_group: Relation([{ sid: "S2", name: "Jones", status: 10 },
//                        { sid: "S3", name: "Blake", status: 30 }]) },
//  { city: "Athens",
//    my_group: Relation([{ sid: "S5", name: "Adams", status: 30 }]) }]

Here is a formatted representation of the result:

+--------+----------------------------+
| :city  | :my_group                  |
+--------+----------------------------+
| London | +------+-------+---------+ |
|        | | :sid | :name | :status | |
|        | +------+-------+---------+ |
|        | | S1   | Smith |      20 | |
|        | | S4   | Clark |      20 | |
|        | +------+-------+---------+ |
|--------+----------------------------+
| Paris  | +------+-------+---------+ |
|        | | :sid | :name | :status | |
|        | +------+-------+---------+ |
|        | | S2   | Jones |      10 | |
|        | | S3   | Blake |      30 | |
|        | +------+-------+---------+ |
|--------+----------------------------+
| Athens | +------+-------+---------+ |
|        | | :sid | :name | :status | |
|        | +------+-------+---------+ |
|        | | S5   | Adams |      30 | |
|        | +------+-------+---------+ |
+--------+----------------------------+