如何連接兩個表（均源自自連接）以創建第三個表？

Question

我正在嘗試解決問題https://sqlzoo.net/wiki/Self_join ，self joins 問題編號 #10，具體說：

查找包含可從 Craiglockhart 到 Lochend 的兩輛巴士的路線。 顯示巴士號碼。 以及首班車的公司、換乘站的名稱和巴士號碼。 和公司的第二輛巴士。'。

我有我的代碼，不知何故它給了我錯誤說：

重復列名 'num'

這是我的代碼：

 SELECT * FROM
    (SELECT *
    FROM route a JOIN route b 
    ON a.company = b.company AND a.num = b.num
    JOIN stops stopa ON (a.stop = stopa.id)
    JOIN stops stopb ON (b.stop = stopb.id)
    WHERE stopa.name = 'Craiglockhart') big  
                              /* [big] table Gives all buses from 
    craiglockhart */

    JOIN


    (SELECT *
    FROM route a JOIN route b 
    ON a.company = b.company AND a.num = b.num
    JOIN stops stopa ON (a.stop = stopa.id)
    JOIN stops stopb ON (b.stop = stopb.id)
    WHERE stopa.name = 'Lochend') small
                              /*[small] Gives all buses from Lochend */
    ON big.b.stop = small.b.stop
                             /*Trying to join the two tables on the basis of 
    the matching values from [big].b.stop field with [small].b.stop */

我不確定這是否會產生預期的結果。 如果是這樣，這是一種有效的方法嗎？ 如果沒有，有人可以幫我寫信並向我解釋它將如何工作，謝謝？

Answer 1

首先，你做了這個： ON a.company = b.company AND a.num = b.num

-> 它為列num返回相同的值 - 所以它是重復的。

使用索引 - 例如： a.num & b.num ：

SELECT a.num
FROM route a JOIN route b 
ON a.company = b.company AND a.num = b.num

--OR

SELECT b.num
FROM route a JOIN route b 
ON a.company = b.company AND a.num = b.num

ON big.b.stop = small.b.stop也會拋出錯誤

正確答案：

SELECT DISTINCT x.num, x.company,x.name,y.num,y.company
FROM (

select a.num as num, a.company as company, stopb.name as name
FROM route a
JOIN route b
ON a.company = b.company AND a.num = b.num
JOIN stops stopa ON (a.stop = stopa.id)
JOIN stops stopb ON (b.stop = stopb.id)
WHERE stopa.name = 'Craiglockhart') x

JOIN

(select a.num as num, a.company as company, stopb.name as name
FROM route a
JOIN route b
ON a.company = b.company AND a.num = b.num
JOIN stops stopa ON (a.stop = stopa.id)
JOIN stops stopb ON (b.stop = stopb.id)
WHERE stopa.name = 'Lochend') y

ON x.name = y.name 

ORDER BY x.num

Answer 2

這是一個被 sqlzoo 標記為“正確答案”的解決方案。

它的工作原理是首先選擇在 Craiglockhart 停止的所有線路，另一方面，選擇所有在 Lochend 停止的線路。 每個搜索都需要兩個 JOIN（停靠點 + 路線）。

最后，該查詢使用帶有 EXISTS 條件的特殊 JOIN 來定位屬於這兩行的所有停靠點。

SELECT
    r1.num,
    r1.company,
    s3.name,
    r2.num,
    r2.company
FROM 
    stops s1
    INNER JOIN route r1 ON r1.stop = s1.id
    INNER JOIN stops s2 ON s2.name = 'Lochend'
    INNER JOIN route r2 ON r2.stop = s2.id
    INNER JOIN stops s3
        ON EXISTS (
            SELECT 1 
            FROM route
            WHERE 
                num = r1.num 
                AND company = r1.company
                AND stop = s3.id
        )
        AND EXISTS (
            SELECT 1 
            FROM route
            WHERE 
                num = r2.num 
                AND company = r2.company
                AND stop = s3.id
        )
WHERE 
    s1.name = 'Craiglockhart'

Answer 3

關於DUPLICATE COLUMN num ：

在自聯接的情況下，您有兩套列（包括列num ）：一個來自route a ，另一個來自route b

(SELECT * -- double set of columns from table [route]
    FROM route a JOIN route b 
    ON a.company = b.company AND a.num = b.num

您必須用字段名稱替換*以忽略字段名稱的重復

例如

(SELECT a.*
    FROM route a JOIN route b 
    ON a.company = b.company AND a.num = b.num

甚至更好 - 不要偷懶，按名稱寫下每個需要的字段