Aliasing

Sometimes you may need to combine queries with the same data type, but possibly with different records. Start by importing:

import scalaql._

// Docs classes
import scalaql.docs.Hogwarts._

Do distinguish those Query inputs, you could use as method to give Query input an alias:

val left = select[Student].as("left")
// left: Query[From[as[Student, "left"]], Student] = FROM(Hogwarts::Student AS "left")

val right = select[Student].as("right")
// right: Query[From[as[Student, "right"]], Student] = FROM(Hogwarts::Student AS "right")

Then you could use them both in one Query.
For instance, let's find students peers:

case class Peer(who: String, age: Int, peer: Option[String])

val peers =
  left
    .leftJoin(right)
    .on(_.age == _.age)
    .map { case (left, rightOpt) =>
      Peer(
        who = left.name,
        age = left.age,
        peer = rightOpt.map(_.name)
      )
    }
// peers: Query[From[as[Student, "left"]] with From[as[Student, "right"]], Peer] = FROM(Hogwarts::Student AS "left") LEFT JOIN FROM(Hogwarts::Student AS "right") -> MAP(MdocSession::App::Peer)

To provide an aliased input, use the same as method on from:

val input = 
  from(students).as("left") &
    from(students).as("right")
// input: From[as[Student, "left"]] with From[as[Student, "right"]] = From[as[+Hogwarts::Student,="left"]] & From[as[+Hogwarts::Student,="right"]]

Then run the Query:

peers
  .show(truncate = false)
  .run(input)
// +--------+---+--------+
// |who     |age|peer    |
// +--------+---+--------+
// |Harry   |19 |Harry   |
// |Ron     |18 |Ron     |
// |Ron     |18 |Hermione|
// |Ron     |18 |Draco   |
// |Hermione|18 |Ron     |
// |Hermione|18 |Hermione|
// |Hermione|18 |Draco   |
// |Draco   |18 |Ron     |
// |Draco   |18 |Hermione|
// |Draco   |18 |Draco   |
// |Cedric  |17 |Cedric  |
// +--------+---+--------+
//

NOTE
Scala 2.12 doesn't support literal types, so you should alias using traits:

trait left

trait right

val left = select[Student].as[left]

val right = select[Student].as[right]