下面的查询可以确定 哪些人可以进行这个项目的第二部分。

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;

说明如下：

在这个查询实例中，演示了怎样从到一个同样的表的连接中查找数据 (p1和p2)，在这里，这种技巧用来确定一对双胞胎是否在 65岁以前就死了，如果是，这一行就不返回。

上述所有信息都存在于所有表中， 我们同时使用id,tvab键 (所有表)，和id,ptvab 键(person_data表) 来使查询更快。

在我们专用的机器上 (A 200MHz UltraSPARC)，这个查询返回 150-200行数据，只需要不到一秒钟。

上面查询的表的当前记录是：

每一次访问以一个称为event的状态码结束，这个查询用来显示一个按 event组合的所有双胞胎的表，这可以指明有多少对双胞胎的调查全部完成，有多少对双胞胎中一个完成一个没有完成，等等。

SELECT
        t1.event,
        t2.event,
        COUNT(*)
FROM
        lentus AS t1,
        lentus AS t2,
        twin_project AS tp
WHERE
        /* We are looking at one pair at a time */
        t1.id = tp.id
        AND t1.tvab=tp.tvab
        AND t1.id = t2.id
        /* Just the screening survey */
        AND tp.survey_no = 5
        /* This makes each pair only appear once */
        AND t1.tvab='1' AND t2.tvab='2'
GROUP BY
        t1.event, t2.event;