在我們的實際專案中，有時候需要寫一下資料彙總和查詢，哪麼子查詢就必不可少。

比如我現在有三張表：

lch_resource_attribute，idx_lch_resource_category，lch_resource_games

那麼我現在要查詢某個應用的子分類的話，用 left join on左連線

select l.game_id as 應用id,l.title as 主分類,m.title as 次分類 from (select * from (SELECT a.game_id, GROUP_CONCAT(b.title) as title FROM `lch_resource_attribute` AS b 
,`idx_lch_resource_category` AS a WHERE ((b.id = a.category_id or b.id =a.parent_id) and level = 1) AND b.status =1 and a.game_status = 1 group by a.game_id order by 
a.game_id asc) as g) as l left join (select * from (SELECT e.game_id, GROUP_CONCAT(d.title) as title FROM `lch_resource_attribute` AS d ,`idx_lch_resource_category` AS 
e WHERE ((d.id = e.category_id or d.id =e.parent_id) and level = 2) AND d.status =1 and e.game_status = 1 group by e.game_id order by e.game_id asc) as k) as m on 
l.game_id = m.game_id order by l.game_id;

結果如下:
+----------+--------------------------------------------------------+------------------------------+
|應用id   | 主分類                                                 | 次分類                       |
+----------+--------------------------------------------------------+------------------------------+
|        1 | 消消樂                                        | NULL                         |
|        3 | 卡牌,撲克也,智,休                      | 休智,閒                |
|        4 | 跑酷                                                   | NULL                         |
|        5 | 卡牌,訊息好                                    | NULL                         |
|        8 | 2.1,仙劍                                           | NULL                         |
|        9 | 2.1,公共                                           | NULL                         |
|       10 | 冒險,籃球                                              | NULL                         |
|       12 | 網遊卡牌,也不錯                  | 卡戲,撲錯          |
|       14 | 冒幻                                          | NULL                         |
+----------+--------------------------------------------------------+------------------------------+

查詢應用基本資訊(包含大小和上線時間),主分類和次分類,這個時候就要先查詢主分類和次分類，然後在查應用屬性。
select k.id, k.name, k.package, k.company, k.developer,k.link,k.size,k.online_time, g.c_title as 主分類,g.s_title as 次分類 from (select * from (SELECT c.id, c.name, 
c.package, c.company, 
c.developer, p.link,p.size, FROM_UNIXTIME(p.update_time) as online_time FROM 
`lch_resource_games` AS c, `idx_lch_resource_version` AS p WHERE c.id = p.game_id AND p.status =1 order by c.id asc) as f) as k, (select * from (select 
l.game_id,l.title as c_title,m.title as s_title from (select * from (SELECT a.game_id, GROUP_CONCAT(b.title) as title FROM `lch_resource_attribute` AS b 
,`idx_lch_resource_category` AS a WHERE (b.id = a.category_id or b.id =a.parent_id) AND b.status =1 and a.game_status = 1 group by a.game_id order by a.game_id asc) as 
g) as l left join (select * from (SELECT e.game_id, GROUP_CONCAT(d.title) as title FROM `lch_resource_attribute` AS d ,`idx_lch_resource_category` AS e WHERE ((d.id = 
e.category_id or d.id =e.parent_id) and level = 2) AND d.status =1 and e.game_status = 1 group by e.game_id order by e.game_id asc) as k) as m on l.game_id = m.game_id 
order by l.game_id) as h) as g where k.id= g.game_id order by k.id desc;
執行的結果如下:
+-----+-----------------+----------------------+------------------+-----------+--------+---------------------+------------------------------+---------------------+
| id  | name            | package              | company          | developer | size   | online_time         | 主分類                       | 次分類              |
+-----+-----------------+----------------------+------------------+-----------+--------+---------------------+------------------------------+---------------------+
| 374 | doufuyy3        | com.doufuyuyue5.am   |

[email protected] | aaaaa      |   0.30 | 2016-03-10 15:20:37 |策,坦克   | 擊,坦克       |
| 373 | 約3       | com.doufuyuyue3.am   | [email protected] | bbbb     |   3.67 | 2016-03-09 18:18:07 |動作,RPG                  | RPG             |
| 372 | 影之刃          | com.yzr.am           | [email protected] | cccc    | 157.88 | 2016-01-19 10:25:37 |略                      | NULL                |
| 371 | 2       | com.doufu.yuyuecc.am | [email protected] | ddddd      |   0.30 | 2016-03-09 10:23:26 |略,RPG            | RPG             |
| 370 | 帶付      | com.doufusdk.am      | [email protected] | eeee      |   4.59 | 2016-03-02 16:16:32 |動,遊戲                  | RPG            |
+-----+-----------------+----------------------+------------------+-----------+--------+---------------------+------------------------------+---------------------+

這樣就查出結果了，整個思路也就出來了。