Question

我有一个如下的SQL查询：

SELECT 
    a.id user_id, a.type, a.email_address, 
    a.name_first, a.name_last, languages.title as language,
    b.created_at purchased_on, b.expires_at watchable_until,
    c.title,
    d.title topic,
    e.title category,
    f.subject, f.grade
FROM users a,
     user_video_purchases b,
     videos c,
     gradesubject_category_topics d,
     gradesubject_categories e,
     gradesubjects f
JOIN languages ON 
          users.language_preferred_id = languages.id
WHERE a.id = b.user_id
      AND b.video_id = c.id
      AND c.gradesubject_category_topic_id = d.id
      AND d.gradesubject_category_id = e.id
      AND e.gradesubject_id = f.id
ORDER BY purchased_on DESC;

此查询返回以下错误消息：

“ on子句”中的未知列“ users.language_preferred_id”

这些列存在于users表中，据我所知JOIN是正确的。

有人可以指出我可能会出问题的地方吗？

Answer 1

您正在混合使用旧式和现代联接语法。您应该只使用后者：

SELECT a.id user_id, a.type, a.email_address, a.name_first, a.name_last,
    l.title as language,
    b.created_at purchased_on,
    b.expires_at watchable_until,
    c.title,
    d.title topic,
    e.title category,
    f.subject, f.grade
FROM users a
INNER JOIN user_video_purchases b
    ON a.id = b.user_id
INNER JOIN videos c
    ON b.video_id = c.id
INNER JOIN gradesubject_category_topics d
    ON c.gradesubject_category_topic_id = d.id
INNER JOIN gradesubject_categories e
    ON d.gradesubject_category_id = e.id
INNER JOIN gradesubjects f
    ON e.gradesubject_id = f.id
INNER JOIN languages l
    ON a.language_preferred_id = l.id
ORDER BY
    purchased_on DESC;

Answer 2

最好使用如下所示的显式联接

SELECT a.id user_id, a.type, a.email_address, a.name_first, a.name_last, languages.title as language,
                                          b.created_at purchased_on, b.expires_at watchable_until,
                                          c.title,
                                          d.title topic,
                                          e.title category,
                                          f.subject, f.grade
                                     FROM users a inner join
                                          user_video_purchases b on a.id = b.user_id
                                          inner join videos c on b.video_id = c.id
                                          inner join gradesubject_category_topics d on c.gradesubject_category_topic_id = d.id
                                          inner join gradesubject_categories e on d.gradesubject_category_id = e.id
                                          inner join gradesubjects f on e.gradesubject_id = f.id
                                          inner JOIN languages l ON a.language_preferred_id = l.id

                                    ORDER BY purchased_on DESC

Answer 3

请停止使用基于逗号的隐式联接，并使用Explicit Join based syntax
使用Proper Aliasing，并在为表加上别名后；您应该仅使用别名>来引用其列。

此外，在多表查询的情况下，始终将列及其表名/别名一起引用。如果在不同的表中有多个具有相同名称的列，这将避免含糊的行为。

这是使用显式联接的重做代码：

SELECT a.id AS user_id, a.type, a.email_address, a.name_first, a.name_last, g.title AS language, b.created_at AS purchased_on, b.expires_at AS watchable_until, c.title, d.title AS topic, e.title AS category, f.subject, f.grade FROM users a JOIN user_video_purchases b ON a.id = b.user_id JOIN videos c ON b.video_id = c.id JOIN gradesubject_category_topics d ON c.gradesubject_category_topic_id = d.id JOIN gradesubject_categories e ON d.gradesubject_category_id = e.id JOIN gradesubjects f ON e.gradesubject_id = f.id JOIN languages g ON a.language_preferred_id = g.id ORDER BY purchased_on DESC;

“ on子句”中的未知列“ xxx”

3 个答案: