union

left.union(right)

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

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’.

Requirements

The headings of the two relations must be identical.

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.minus(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

SELECT `t1`.`product_id`, `t1`.`quantitiy`
FROM `my_purchases` AS 't1'
UNION
SELECT `t1`.`product_id`, `t1`.`quantitiy`
FROM `your_purchases` AS 't1'

Note that the scoping of the table alias t1 is limited to the respective SELECT clauses, which is why the same alias is used in both clauses.