如果没有行,如何按计数分组?

例如,表 category_description

name    | category_id | nav_id
History | 62          |  1
Romance | 61          |  1
mysql   | 59          |  1
mssql   | 60          |  1

然后 product_to_category 具有以下行,

product_id | category_id
 55        |   62
 54        |   62

select name,count(a.category_id) from category_description a, 
product_to_category b 
where a.category_id = b.category_id group by name;

我得到以下结果

+---------+----------------------+
| name    | count(a.category_id) |
+---------+----------------------+
| History |                    2 |
+---------+----------------------+

但是我试图计算空数据,例如除了以下输出之外

+---------+----------------------+
| name    | count(a.category_id) |
+---------+----------------------+
| History |                     2|
|  mysql  |                     0|
|  mssql  |                     0|
|Romance  |                     0|
+---------+----------------------+