Inserts if not exists or select in one-trip to database

inselect.sql

--sql, args := DB.
--    Insect("tab").
--    Columns("b", "c").
--    Values(1, 2).
--    Where("d = $1", 3).
--    Returning("id", "f", "g").
--    ToSQL()

WITH
    sel AS (SELECT id, f, g FROM tab WHERE (d = $1)),
    ins AS (
        INSERT INTO "tab"("b","c")
        SELECT $2,$3
        WHERE NOT EXISTS (SELECT 1 FROM sel)
        RETURNING "id","f","g"
    )
SELECT * FROM ins UNION ALL SELECT * FROM sel