Question

我正在尝试获取MySQL查询，以根据参数返回城市页面上的一些活动：

该城市的短片
choosem CategoryID＆＃39; s

MySQL查询：

SELECT
                a.ActivityID as ActivityID,
                a.Name as ActivityName,
                a.Description as ActivityDescription,
                a.Address as ActivityAddress,
                a.Mail as ActivityMail,
                a.Shorttag as ActivityShorttag,
                a.Phone as ActivityPhone,
                (SELECT ap.ThumbPath FROM ActivityPictures ap WHERE Acti_ActivityID = a.ActivityID ORDER BY MainPicture DESC, PictureID ASC LIMIT 1) as ActivityThumbPath,
                c.CityID as CityID,
                c.Name as CityName,
                c.PostalCode as CityPostalCode,
                c.Shorttag as CityShorttag,
                c.ShortDescription as CategoryShortDescription,
                c.LongDescription as CategoryLongDescription,
                ca.CategoryID as CategoryID
            FROM
                Cities c
            LEFT JOIN
                Activities a
                ON
                    c.CityID = a.City_CityID
            LEFT JOIN
                ActivityCategoryConn acc
                ON
                    a.ActivityID = acc.Acti_ActivityID
            LEFT JOIN
                Categories ca
                ON
                    acc.Cate_CategoryID = ca.CategoryID
                    AND
                    ca.CategoryID IN (2)
            WHERE
                c.Shorttag = 'city-shorttag'
            ORDER BY
                a.ActivityID desc

我希望这总是至少返回一行来自＆＃34; Cities c＆＃34; - 但如果没有活动与CategoryID匹配，则其余活动只能为NULL。如果有活动，这些当然应该返回匹配的行数。

如何使此查询始终返回至少一条包含c。*的记录 - 即使＆＃34; IN＆＃34; -statement中的数组不包含匹配的CategoryID？

Answer 1

您可以通过使用IS NULL语句进行ORing来回退到空值

AND
(
  ca.CategoryID IN (2)
  OR
  ca.CategoryID IS NULL
)

MySQL加入 - 确保始终找到一条记录

1 个答案: