Case

Search for the first condition that evaluates to true and return its associated value. If none of the conditions match, null is returned.

from employees
derive distance = case [
  city == "Calgary" => 0,
  city == "Edmonton" => 300,
]

SELECT
  *,
  CASE
    WHEN city = 'Calgary' THEN 0
    WHEN city = 'Edmonton' THEN 300
    ELSE NULL
  END AS distance
FROM
  employees

To set a default, a true condition can be used:

from employees
derive distance = case [
  city == "Calgary" => 0,
  city == "Edmonton" => 300,
  true => "Unknown",
]

SELECT
  *,
  CASE
    WHEN city = 'Calgary' THEN 0
    WHEN city = 'Edmonton' THEN 300
    ELSE 'Unknown'
  END AS distance
FROM
  employees