如何将多列数据合并为一个？ [邻接表模型查询]

Question

我有此查询

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; 

这是怎么造成像

看起来你可以看到它返回四列。 我想将所有四个不同的数据合并成一列。

我看了类似的问题，但没有一个在他们的查询中留下了联接。 我知道它可以使用union但我挣扎。

谢谢。

Answer 1

正如您所说，您可以在查询结果上使用UNION。在剩下的答案中，我将向您展示一个使用CTE的解决方案，CTE是许多系统中可用的标准SQL语法，但不幸的是，不在MySQL中。但是，要将这个查询转换为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;