使用 IFNULL 将 NULL 设置为零

Question

我有一个表，其中的字段包含整数或 NULL。

parent_id
2
4
6
NULL
NULL
45
2

我该如何添加 IFNULL 语句，以便用 0 而不是 NULL 填充 ans_count ？

这是我的 SQL 代码：

...
(SELECT parent_id AS pid, COUNT(*) AS ans_count
   FROM qa
  GROUP BY parent_id) AS n

更新

下面是完整的 SQL - 感谢大家的耐心等待。

SELECT  *
FROM    qa
        JOIN user_profiles
          ON user_id = author_id
        LEFT JOIN (SELECT cm_id,
                          cm_author_id,
                          id_fk,
                          cm_text,
                          cm_timestamp,
                          first_name AS cm_first_name,
                          last_name AS cm_last_name,
                          facebook_id AS cm_fb_id,
                          picture AS cm_picture
                    FROM  cm
                    JOIN  user_profiles
                      ON  user_id = cm_author_id) AS c
          ON id = c.id_fk
        LEFT JOIN (SELECT   parent_id AS pid, COUNT(*) AS ans_count
                     FROM   qa
                    GROUP   BY parent_id) AS n
          ON id = n.pid
WHERE   id  LIKE '%'
ORDER   BY id DESC

Answer 1

编辑：基于完整查询的新信息

您指定的查询中计数可能为空的原因是因为左联接将在不匹配的记录上返回空值。因此子查询本身不会返回空计数（因此出现所有响应和混乱）。您需要在最外层的 select 中指定 IFNULL，如下所示：

SELECT  qa.*, user_profiles.*, c.*, n.pid, ifnull(n.ans_count, 0) as ans_count
FROM    qa
        JOIN user_profiles
          ON user_id = author_id
        LEFT JOIN (SELECT cm_id,
                          cm_author_id,
                          id_fk,
                          cm_text,
                          cm_timestamp,
                          first_name AS cm_first_name,
                          last_name AS cm_last_name,
                          facebook_id AS cm_fb_id,
                          picture AS cm_picture
                    FROM  cm
                    JOIN  user_profiles
                      ON  user_id = cm_author_id) AS c
          ON id = c.id_fk
        LEFT JOIN (SELECT   parent_id AS pid, COUNT(*) AS ans_count
                     FROM   qa
                    GROUP   BY parent_id) AS n
          ON id = n.pid
WHERE   id  LIKE '%'
ORDER   BY id DESC

OLD RESPONSE

您能否更详细地解释一下您所看到的以及您期望看到的内容？计数不能返回 NULL。

运行这组查询，您将看到计数始终为 2。您可以更改 NULL Parent_ids 的显示方式（如 NULL 或 0），但计数本身将始终返回。

create temporary table if not exists SO_Test(
    parent_id int null);

insert into SO_Test(parent_id)
select 2 union all select 4 union all select 6 union all select null union all select null union all select 45 union all select 2;


SELECT IFNULL(parent_id, 0) AS pid, COUNT(*) AS ans_count
   FROM SO_Test
  GROUP BY IFNULL(parent_id, 0);

SELECT parent_id AS pid, COUNT(*) AS ans_count
   FROM SO_Test
  GROUP BY parent_id;

drop table SO_Test;

Answer 2

我没有测试这个，但我认为它会起作用

(SELECT IF( parent_id IS NULL, 0, parent_id) AS pid, COUNT(*) AS ans_count
   FROM qa
  GROUP BY parent_id) AS n

Answer 3

只需将其包含在您的声明中即可：

IFNULL( 
  (SELECT parent_id AS pid, COUNT(*) AS ans_count
   FROM qa
   GROUP BY parent_id)
 , 0
) AS n

Answer 4

您是否尝试过仅计算parent_id？

(SELECT parent_id AS pid, COUNT(parent_id) AS ans_count
   FROM qa
  GROUP BY parent_id)

Answer 5

SELECT IFNULL(parent_id, 0) AS pid, COUNT(IFNULL(parent_id, 0)) AS ans_count
FROM qa
GROUP BY IFNULL(parent_id, 0)

Answer 6

您可以发布展示您正在谈论的行为的实际数据和完整查询吗？根据我的经验，COUNT(*) 永远不能为 NULL。

Count(*) 可以返回 null 吗？

COUNT(*) 总是返回结果吗？