Question

我正在执行以下脚本：

SELECT  a.*,
    gg.image as 'recent_image_file' 
FROM 

(   SELECT  c.cid, 
            c.category_name AS 'cat_name',
            COUNT(g.id) AS image_count,
            c.category_image AS 'images',
            c.estgendpref,
            MAX(n.id) as 'recent_image'
    FROM tbl_category c
    LEFT JOIN tbl_gallery_category g 
        ON c.cid=g.cat_id
    LEFT JOIN tbl_gallery n 
        ON n.id=g.image_id

    WHERE n.date_active < NOW()
    GROUP BY c.cid
    ORDER BY c.category_name ASC
) a

INNER JOIN tbl_gallery gg
    ON a.recent_image = gg.id;

除了MAX（n.id）忽略WHERE子句并返回类别的最高id之外，一切都返回正常。如何让MAX（n.id）返回结果集的最大值，包括WHERE子句？感谢。

Answer 1

你应该在on子句中使用where条件添加一个正确的AND ..否则，where where work as inner join condition

SELECT  a.*,
    gg.image as 'recent_image_file' 
FROM 

(   SELECT  c.cid, 
            c.category_name AS 'cat_name',
            COUNT(g.id) AS image_count,
            c.category_image AS 'images',
            c.estgendpref,
            MAX(n.id) as 'recent_image'
    FROM tbl_category c
    LEFT JOIN tbl_gallery_category g  ON c.cid=g.cat_id
    LEFT JOIN tbl_gallery n ON n.id=g.image_id and n.date_active < NOW()
    GROUP BY c.cid
    ORDER BY c.category_name ASC
) a

INNER JOIN tbl_gallery gg
    ON a.recent_image = gg.id;

Answer 2

因此，感谢那些建议的人，我发现正在使用WHERE子句。然而，MAX（id）并不是真正想要的，因为较低的id可能有更大的日期。以下作品很有魅力。

def __init__(self):
    # http://stackoverflow.com/a/4140988/675216
    vcmd= (self.root.register(self.validate_spin), '%W', '%P')
    # Rest of code left out
    self.spin.config(validate="key", validatecommand=vcmd)
    self.spin.bind("<<ResetValidate>>", self.on_reset_validate)

def on_reset_validate(self, event):
    # Turn validate back on and set textVariable
    self.spin.config(validate="key")

def validate_spin(self, name, nv):
    # Do validation ...
    if not valid:
        self.spin.event_generate("<<ResetValidate>>", when="tail")
    return valid

MySQL - 同一查询中的COUNT和MAX。 MAX忽略WHERE子句

2 个答案: