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Relational Algebra for Modern Times

union

left.union(right)

Problem

Section titled “Problem”

Given two relations with the same heading, create a relation with all tuples from both relations.

Example: I want the list of all managers plus all employees.

Description

Section titled “Description”

The set union of two relations. The result contains all tuples from left as well as all tuples from right.

The result’s heading is identical with that of the inputs’. Duplicate tuples are automatically removed (since relations are sets).

Requirements

Section titled “Requirements”

The headings of the two relations must be identical.

Example

Section titled “Example”
my_purchases = Bmg::Relation.new([
  { product_id: 10, quantity: 2 },
  { product_id: 10, quantity: 4 },
  { product_id: 20, quantity: 1 },
])


your_purchases = Bmg::Relation.new([
  { product_id: 10, quantity: 2 },
  { product_id: 10, quantity: 4 },
  { product_id: 20, quantity: 5 },
  { product_id: 30, quantity: 1 },
])


my_purchases.union(your_purchases).to_a


=>
[{ product_id: 10, quantity: 2 },
 { product_id: 10, quantity: 4 },
 { product_id: 20, quantity: 1 },
 { product_id: 20, quantity: 5 },
 { product_id: 30, quantity: 1 }]
Generated SQL
Section titled “Generated SQL”
SELECT `t1`.`product_id`, `t1`.`quantity`
FROM `my_purchases` AS 't1'
UNION
SELECT `t1`.`product_id`, `t1`.`quantity`
FROM `your_purchases` AS 't1'