首先你需要
UNION ALL articles 表和 video 表必须是子查询，然后使用 Outer join 以及 COUNT 功能。

SELECT Concat('Category ID  ', c.id, ' is use ', Count(t.category_id ), ' times') 
FROM   categories c 
       LEFT JOIN (SELECT category_id 
                  FROM   articles 
                  UNION ALL 
                  SELECT category_id 
                  FROM   video) t 
              ON c.id = t.category_id 
GROUP  BY c.id

sqlfiddle:http：//sqlfiddle.com/#！2012年9月9日
[结果]：

| Concat('Category ID  ', t.id, ' is use ', t.cnt, ' times') |
|------------------------------------------------------------|
|                              Category ID  1 is use 3 times |
|                              Category ID  2 is use 2 times |
|                              Category ID  3 is use 3 times |
|                              Category ID  4 is use 0 times |

笔记 COUNT 如果列值遇到 null 例如，这里有一个示例脚本。

CREATE TABLE T(
   col int
);

INSERT INTO T VALUES (NULL);
INSERT INTO T VALUES (1);

SELECT COUNT(col) FROM  t;  -- RESULT  = 1

SELECT COUNT(*) FROM  t;  --RESULT = 2

样品sqlfiddle:http：//sqlfiddle.com/#！9/e2bba7/2号

例如，您可以使用以下查询：

Select category_id, count(1) - 1
  From (
    Select category_id From video
    Union All Select category_id From articles
    Union All Select id From Categories)
  Group By category_id