SQL indsæt i vælg fra flere felter

Du kan bruge en selvtilmelding:

INSERT INTO jobs
  (customer_id, client_id)
SELECT customer.id, client.id
FROM   users customer
  JOIN users client ON customer.username = ? AND client.username = ?

Eller du kan bruge underforespørgsler:

INSERT INTO jobs
  (customer_id, client_id)
VALUES (
  (SELECT id FROM users WHERE username = ?),
  (SELECT id FROM users WHERE username = ?)
)