La siguiente consulta es empleada para determinar quiénes pasan a la segunda parte del proyecto:

SELECT
    CONCAT(p1.id, p1.tvab) + 0 AS tvid,
    CONCAT(p1.christian_name, ' ', p1.surname) AS Name,
    p1.postal_code AS Code,
    p1.city AS City,
    pg.abrev AS Area,
    IF(td.participation = 'Aborted', 'A', ' ') AS A,
    p1.dead AS dead1,
    l.event AS event1,
    td.suspect AS tsuspect1,
    id.suspect AS isuspect1,
    td.severe AS tsevere1,
    id.severe AS isevere1,
    p2.dead AS dead2,
    l2.event AS event2,
    h2.nurse AS nurse2,
    h2.doctor AS doctor2,
    td2.suspect AS tsuspect2,
    id2.suspect AS isuspect2,
    td2.severe AS tsevere2,
    id2.severe AS isevere2,
    l.finish_date
FROM
    twin_project AS tp
    /* For Twin 1 */
    LEFT JOIN twin_data AS td ON tp.id = td.id
              AND tp.tvab = td.tvab
    LEFT JOIN informant_data AS id ON tp.id = id.id
              AND tp.tvab = id.tvab
    LEFT JOIN harmony AS h ON tp.id = h.id
              AND tp.tvab = h.tvab
    LEFT JOIN lentus AS l ON tp.id = l.id
              AND tp.tvab = l.tvab
    /* For Twin 2 */
    LEFT JOIN twin_data AS td2 ON p2.id = td2.id
              AND p2.tvab = td2.tvab
    LEFT JOIN informant_data AS id2 ON p2.id = id2.id
              AND p2.tvab = id2.tvab
    LEFT JOIN harmony AS h2 ON p2.id = h2.id
              AND p2.tvab = h2.tvab
    LEFT JOIN lentus AS l2 ON p2.id = l2.id
              AND p2.tvab = l2.tvab,
    person_data AS p1,
    person_data AS p2,
    postal_groups AS pg
WHERE
    /* p1 gets main twin and p2 gets his/her twin. */
    /* ptvab is a field inverted from tvab */
    p1.id = tp.id AND p1.tvab = tp.tvab AND
    p2.id = p1.id AND p2.ptvab = p1.tvab AND
    /* Just the screening survey */
    tp.survey_no = 5 AND
    /* Skip if partner died before 65 but allow emigration (dead=9) */
    (p2.dead = 0 OR p2.dead = 9 OR
     (p2.dead = 1 AND
      (p2.death_date = 0 OR
       (((TO_DAYS(p2.death_date) - TO_DAYS(p2.birthday)) / 365)
        >= 65))))
    AND
    (
    /* Twin is suspect */
    (td.future_contact = 'Yes' AND td.suspect = 2) OR
    /* Twin is suspect - Informant is Blessed */
    (td.future_contact = 'Yes' AND td.suspect = 1
                               AND id.suspect = 1) OR
    /* No twin - Informant is Blessed */
    (ISNULL(td.suspect) AND id.suspect = 1
                        AND id.future_contact = 'Yes') OR
    /* Twin broken off - Informant is Blessed */
    (td.participation = 'Aborted'
     AND id.suspect = 1 AND id.future_contact = 'Yes') OR
    /* Twin broken off - No inform - Have partner */
    (td.participation = 'Aborted' AND ISNULL(id.suspect)
                                  AND p2.dead = 0))
    AND
    l.event = 'Finished'
    /* Get at area code */
    AND SUBSTRING(p1.postal_code, 1, 2) = pg.code
    /* Not already distributed */
    AND (h.nurse IS NULL OR h.nurse=00 OR h.doctor=00)
    /* Has not refused or been aborted */
    AND NOT (h.status = 'Refused' OR h.status = 'Aborted'
    OR h.status = 'Died' OR h.status = 'Other')
ORDER BY
    tvid;

Algunas explicaciones:

Esta consulta muestra, entre otras cosas, cómo realizar búsquedas en una tabla a partir de la misma tabla con una unión (p1 y p2). En el ejemplo, esto se usa para verificar cuándo una pareja de gemelos murieron antes de cumplir los 65 años. Si sucede eso, la fila no se devuelve.

Todo lo mencionado anteriormente existe en todas las tablas con información relativa a gemelos. Hay un índice definido sobre los campos id,tvab (en todas las tablas) y sobre id,ptvab (person_data) para realizar las consultas más rápidamente.

En nuestra máquina de producción (un UltraSPARC a 200Mhz), esta consulta devuelve cerca de 150-200 filas y toma menos de un segundo.

La cantidad actual de registros en las tablas usadas en la consulta: