遇到的問題：
現在利用SQL語句查詢得到三個結果集，每一個結果集是兩列，具體為排名序號與對應的名字，如下表一。但是現在需要將這三個結果集拼接成為一個表，並且只有一個排名序號。
| 排名 | 姓名 |
| 1 | 張三 |
| 2 | 李四 |
利用SQL語句實現：
SELECT rownum, MAX(post1) AS post1,MAX(post2) AS post2,MAX(post3) AS post3
FROM
(SELECT @rownum1 := @rownum1 + 1 AS rownum, firstname AS post1,'' AS post2,'' AS post3
FROM
(SELECT firstname
FROM
exam_main a,
exam_rel_user b,
account c,
core_dept d
WHERE a.`id` = b.`exam_id`
AND b.`user_id` = c.`user_id`
AND c.`dept_id` = d.`id`
AND c.`position` IN (
'01010101',
'01010102',
'01010104'
)
ORDER BY b.score DESC,
b.last_attempt_date ASC) aa,
(SELECT
@rownum1 := 0) bb
UNION ALL
SELECT @rownum2 := @rownum2 + 1 AS rownum, '' AS post1,firstname AS post2, '' AS post3
FROM
(SELECT
firstname
FROM
exam_main a,
exam_rel_user b,
account c,
core_dept d
WHERE a.`id` = b.`exam_id`
AND b.`user_id` = c.`user_id`
AND c.`dept_id` = d.`id`
AND c.`position` IN (
'01010201',
'01010202',
'01010204'
)
ORDER BY b.`score` DESC,
b.`last_attempt_date` ASC) aa,
(SELECT
@rownum2 := 0) bb
union all
SELECT @rownum3 := @rownum3 + 1 AS rownum,'' as post1, '' as post2, firstname as post3 //這里的兩個單引號分別代表兩個空值，也就是把post1與post2值設為空，最后也就是把第一列與第二列的設為空的一列。
from
(SELECT
firstname
FROM
exam_main a,
exam_rel_user b,
account c,
core_dept d
WHERE a.`id` = b.`exam_id`
AND b.`user_id` = c.`user_id`
AND c.`dept_id` = d.`id`
AND c.`position` IN ('01010203', '01010206')
ORDER BY b.`score` DESC,
b.`last_attempt_date` ASC) aa,
(SELECT
@rownum3 := 0) bb
) cc
/* where rownum = 1*/
group by rownum
注：
1.這里面涉及四個表exam_main a, exam_rel_user b, account c, core_dept d ，其中考試記錄表exam_main中的id為考試結果表exam_rel_user的exam_id外鍵，考試結果表中user_id外鍵為人員表中user_id，人員表中dept_id外鍵為core_dept部門表中的id。
2.每個結果集看作是一個表，三個結果集用 union all 連接，最后將三個結果集又看做一個表。
3.用SQL語句產生排名序號利用的是在結果集（當做一個表）之外添加查詢.如下：
SELECT @rownum := @rownum + 1 AS rownum,'' as post1, '' as post2, firstname as post3
FROM （結果集）aa
(SELECT @rownum3 := 0) bb
此處勿忘在最后的括號外添加別名，否則會出錯。