Holas, fjbanegas.
Para eso utiliza la instrucción UNION:
Código SQL:
Ver originalSELECT
`id`
FROM
(
SELECT id_alumno AS `id` FROM alumno
UNION
SELECT id_catedratico AS `id` FROM catedratico
UNION
SELECT id_admin AS `id` FROM admin
)
Y si quieres, hasta puedes dejar señalado de dónde es cada ID:
Código SQL:
Ver originalSELECT
`id`,
origen
FROM
(
SELECT id_alumno AS `id`, 'alumno' AS origen FROM alumno
UNION
SELECT id_catedratico AS `id`, 'catedratico' AS origen FROM catedratico
UNION
SELECT id_admin AS `id`, 'admin' AS origen FROM admin
)
Saludos!