An example of a CTE in SQL which will either SELECT a column from a table, or insert a new column into that table.

SelectOrInsert.sql

WITH sel AS (
        SELECT id FROM hometowns WHERE name = 'Portland'
        ), 
  ins AS (
        INSERT INTO hometowns(name)
        SELECT 'Portland'
        WHERE NOT EXISTS (SELECT 1 FROM sel)
        RETURNING id
)
INSERT INTO users(name, hometown_id)
    VALUES ('Robert', SELECT id FROM ins UNION ALL SELECT id FROM sel);