具有3个表的复杂mysql查询

now i have three tables ok?

1. catgories | cat_id
2. areas | area_id
3. ads | cat_id - area_id

now i want get the the areas under the catgories with contain the ads this mean - catgorty -- area contant catgorty ads example i have cars as a catgory and egypt as area and a car for sale as ads now i want show the areas under the catgories which contain ads this mean i have egypt usa canda and one ads in egypt now i want show it like cars -- egypt and if i have one more ad in the usa it appear cars --egypt --usa this mean give me the areas with contain ads under that catgory

countries with adds:

SELECT *
  FROM areas
 WHERE area_id in (SELECT area_id FROM ads) 

countries with adds for cars:

SELECT *
  FROM areas
 WHERE area_id in (
    SELECT ads.area_id
      FROM ads,categories
     WHERE ads.cat_id = categories.cat_id
       AND categories.cat_name = 'cars') 

is that what you meant?

You are looking for GROUP BY:

SELECT cat_id, group_concat(area_id) from ads group by cat_id;

Assuming *cat_id* is the actual value (cars) and not an auto_increment. If so, use INNER JOIN.