Sisipkan SQL ke pilih dari beberapa bidang

Anda bisa menggunakan self-join:

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

Atau, Anda dapat menggunakan subkueri:

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