如何將多列數據合併爲一個？ [鄰接表模型查詢]

Question

我有此查詢

SELECT t1.categoryid AS lev1, t2.categoryid as lev2, t3.categoryid as lev3, t4.categoryid as lev4 
FROM category AS t1 
LEFT JOIN category AS t2 ON t2.parent = t1.categoryid 
LEFT JOIN category AS t3 ON t3.parent = t2.categoryid 
LEFT JOIN category AS t4 ON t4.parent = t3.categoryid 
WHERE t1.categoryid = 4149418031; 

這是怎麼造成像

看起來你可以看到它返回四列。 我想將所有四個不同的數據合併成一列。

我看了類似的問題，但沒有一個在他們的查詢中留下了聯接。 我知道它可以使用union但我掙扎。

謝謝。

Answer 1

正如您所說，您可以在查詢結果上使用UNION。在剩下的答案中，我將向您展示一個使用CTE的解決方案，CTE是許多系統中可用的標準SQL語法，但不幸的是，不在MySQL中。但是，要將這個查詢轉換爲MySQL，你可以看看下面的答案：How do you use the "WITH" clause in MySQL?

WITH query AS (
    SELECT t1.categoryid AS lev1, t2.categoryid as lev2, t3.categoryid as lev3, t4.categoryid as lev4 
    FROM category AS t1 
    LEFT JOIN category AS t2 ON t2.parent = t1.categoryid 
    LEFT JOIN category AS t3 ON t3.parent = t2.categoryid 
    LEFT JOIN category AS t4 ON t4.parent = t3.categoryid 
    WHERE t1.categoryid = 4149418031) 
SELECT lev1 AS category_value FROM query 
    UNION 
SELECT lev2 AS category_value FROM query 
    UNION 
SELECT lev3 AS category_value FROM query 
    UNION 
SELECT lev4 AS category_value FROM query;