# `Selecto.SetOperations`

Set operations for combining query results using UNION, INTERSECT, and EXCEPT.

Set operations allow combining results from multiple Selecto queries using
standard SQL set operations. All participating queries must have compatible
column counts and types.

## Examples

    # Basic UNION - combine results from two queries
    query1 = Selecto.configure(users_domain, connection)
      |> Selecto.select(["name", "email"])
      |> Selecto.filter([{"active", true}])

    query2 = Selecto.configure(contacts_domain, connection)
      |> Selecto.select(["full_name", "email_address"])
      |> Selecto.filter([{"status", "active"}])

    combined = Selecto.union(query1, query2, all: true)

    # INTERSECT - find common records
    premium_active = Selecto.intersect(premium_users, active_users)

    # EXCEPT - find differences
    free_users = Selecto.except(all_users, premium_users)

    # Chained set operations
    result = query1
      |> Selecto.union(query2)
      |> Selecto.intersect(query3)
      |> Selecto.except(query4)

# `except`

Create an EXCEPT set operation between two queries.

Returns rows from the first query that don't appear in the second query.

## Options

- `:all` - Use EXCEPT ALL to include duplicates in difference (default: false)
- `:column_mapping` - Map columns between incompatible schemas

# `intersect`

Create an INTERSECT set operation between two queries.

Returns only rows that appear in both queries.

## Options

- `:all` - Use INTERSECT ALL to include duplicate intersections (default: false)
- `:column_mapping` - Map columns between incompatible schemas

# `union`

Create a UNION set operation between two queries.

## Options

- `:all` - Use UNION ALL to include duplicates (default: false)
- `:column_mapping` - Map columns between incompatible schemas

## Examples

    # Basic UNION (removes duplicates)
    Selecto.union(query1, query2)

    # UNION ALL (includes duplicates, faster)
    Selecto.union(query1, query2, all: true)

    # UNION with column mapping
    Selecto.union(customers, vendors,
      column_mapping: [
        {"name", "company_name"},
        {"email", "contact_email"}
      ]
    )

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*Consult [api-reference.md](api-reference.md) for complete listing*