Question

为什么这在MySQL中不起作用？

SELECT blogentry.*, 
       person.personName, 
       (SELECT * 
          FROM BlogEntryComment 
         Where BlogEntryID = '8') as CommentCount    
  FROM blogentry 
 INNER JOIN person ON blogentry.personID = person.personID
 WHERE blogentry.deleted = 'N'
ORDER BY blogentry.dateAdded DESC

Answer 1

子查询只需要返回一个值：字段计数。 *返回所有行，而count(*)将返回有多少行。

(SELECT count(*) FROM BlogEntryComment Where BlogEntryID = '8')

Answer 2

你必须使用聚合函数COUNT来获取值 - 在subSELECT中SELECT *将是脆弱的，因为它试图将行的所有列值返回到单个列中

也就是说，您所拥有的内容将为返回的每条CommentCount记录返回相同的BLOGENTRY值。以下是更好的方法：

   SELECT be.*,
          p.personname,
          COALESCE(x.num, 0) AS CommentCount
     FROM BLOGENTRY be
     JOIN PERSON p ON p.personid = be.personid
LEFT JOIN (SELECT bec.blogentryid,
                  COUNT(*) AS num
             FROM BLOGENTRYCOMMENT bec
         GROUP BY bec.blogentryid) x ON x.blogentryid = be.blogentryid

MySQL查询帮助，包括计数？

2 个答案: