SQL - 计数不返回零

Question

我正在运行此查询，它从一个表中选择几个字段并从其他表中返回计数。

我的问题：如果我的计数器字段之一计数为零，它不会返回数字0，它只是空白 - 这是我想尝试和解决的。

SELECT mem.member_id, mem.screen_name, mem.firstname, mem.lastname, mem.country_code, 
mem.joined, rep.rep as reputation, com.cnt as comments FROM members AS mem 
LEFT OUTER JOIN (
    SELECT member_id, SUM(awarded_what) as rep 
    FROM members_reputation 
    GROUP BY member_id) rep 
    ON mem.member_id = rep.member_id 
LEFT OUTER JOIN (
    SELECT member_id, COUNT(comment_id) as cnt 
    FROM blog_comments 
    GROUP BY comment_id) com 
    ON mem.member_id = com.member_id 
GROUP BY mem.member_id 
ORDER BY mem.joined DESC 

我想什么是这样的：

Screen Name  | Comments 
-----------------|-------------------- 
marty76   | 0 
jonnyBoy12  | 0 
adamApple  | 12 

但是，我得到这样的事情，而不是！

Screen Name  | Comments 
-----------------|-------------------- 
marty76   | 
jonnyBoy12  | 
adamApple  | 12 

使用我的服务器端语言，通过用零替换空值可以很容易地解决这个问题。但是我想要直接从SQL获得零，所以我可以通过计数来排序。

任何建议将是奇妙的。

Answer 1

尝试IFNULL功能

SELECT mem.member_id, mem.screen_name, mem.firstname, mem.lastname, mem.country_code, 
mem.joined, rep.rep as reputation, IFNULL(com.cnt,0) as comments FROM members AS mem 
LEFT OUTER JOIN ( 
    SELECT member_id, SUM(awarded_what) as rep 
    FROM members_reputation 
    GROUP BY member_id) rep 
    ON mem.member_id = rep.member_id 
LEFT OUTER JOIN ( 
    SELECT member_id, COUNT(comment_id) as cnt 
    FROM blog_comments 
    GROUP BY comment_id) com 
    ON mem.member_id = com.member_id 
GROUP BY mem.member_id 
ORDER BY mem.joined DESC 

Answer 2

你能使用ISNULL？

ISNULL(com.cnt, 0) as comments 

Answer 3

变化：

COUNT(comment_id) 

到

IFNULL(COUNT(comment_id), 0) 

Answer 4

其他的方式可以是：

coalesce(count(comment_id),0) 

Answer 5

我想这应该给你你想要得到的结果它比你的查询简单得多：

SELECT mem.member_id, mem.screen_name, mem.firstname, mem.lastname, 
    mem.country_code, mem.joined, 
    SUM(awarded_what) reputation, COUNT(comment_id) comments 
FROM members mem 
LEFT JOIN members_reputation rep ON mem.member_id = rep.member_id 
LEFT JOIN blog_comments com ON mem.member_id = com.member_id 
GROUP BY mem.member_id 
ORDER BY mem.joined DESC