MySQL查詢幫助，包括計數？

Question

任何想法，爲什麼這不起作用在MySQL？

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

子查詢只需要返回一個值：字段數。 *返回所有行，而count(*)將返回有多少行。

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

Answer 2

你必須使用聚合函數COUNT爲了得到價值 - 子查詢中會體弱SELECT *，因爲它正試圖爲行返回所有的列值到一個列。

也就是說，你所擁有的將返回相同的CommentCount值，每個BLOGENTRY記錄返回。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