父母子女的ID

Question

加入試想一下，你有一個像這樣的表：

+-------------+------------------+------+-----+---------+----------------+ 
| Field  | Type    | Null | Key | Default | Extra   | 
+-------------+------------------+------+-----+---------+----------------+ 
| id   | int(11)   | NO | PRI | NULL | auto_increment | 
| name  | varchar(255)  | NO |  |   |    | 
| parent_id | int(11)   | YES | MUL | NULL |    | 
+-------------+------------------+------+-----+---------+----------------+ 

讓我們把這個表locations

這代表着一個城市或國家。

例如，如果name字段是Los Angeles，則其parent_id將代表具有name字段California的行。

現在，假設你有另一個表是這樣的：

+-----------------+---------------+------+-----+---------+----------------+ 
| Field   | Type   | Null | Key | Default | Extra   | 
+-----------------+---------------+------+-----+---------+----------------+ 
| id    | int(11)  | NO | PRI | NULL | auto_increment | 
| name   | varchar(450) | YES |  | NULL |    | 
| state   | varchar(135) | YES |  | NULL |    | 
+-----------------+---------------+------+-----+---------+----------------+ 

讓我們把這個表cities。

每一行代表一個城市，id字段與locations表中的id匹配。

在此表中，state字段始終爲空，因此我想用locations表中的name字段對其進行更新。

我已經試過爲了得到state值此查詢，但它似乎不工作（這需要很長的時間，沒有任何反應）：

SELECT name FROM locations WHERE id IN 
(SELECT parent_gid FROM locations INNER JOIN cities 
ON locations.id = cities.id); 

如何實現任何建議這個？

Answer 1

如果僅僅只是想SELECT語句，使用JOIN

SELECT a.id, a.name, b.name As State 
FROM cities a 
     INNER JOIN locations b 
      ON a.id = b.id 

但如果你想更新表，

UPDATE cities a 
     INNER JOIN locations b 
      On a.ID = b.ID 
SET  a.State = b.name 

UPDATE

SELECT a.id, a.name, c.name As State 
FROM cities a 
     INNER JOIN locations b 
      ON a.id = b.id 
     LEFT JOIN locations c 
      On b.parent_ID = c.id 

Answer 2

在這裏你去：我不知道如何保守並鏈接在SQL小提琴 - 但這裏是代碼：

 
create table loca (
    locaid int, 
    locaname varchar(25), 
    locaparent varchar(25)); 

create table cityb (
    citybid int, 
    citybname varchar(25), 
    citybstate varchar(25)); 

insert into loca (locaid, locaname, locaparent) 
    values (1, 'name1', 'parent1'); 
insert into loca (locaid, locaname, locaparent) 
    values (2, 'name2', 'parent2'); 
insert into loca (locaid, locaname, locaparent) 
    values (3, 'name3', 'parent3'); 
insert into cityb (citybid, citybname, citybstate) 
    values (1, 'city1', ''); 
insert into cityb (citybid, citybname, citybstate) 
    values (2, 'city2', ''); 
insert into cityb (citybid, citybname, citybstate) 
    values (3, 'city3', ''); 

update loca, cityb 
    set citybstate = locaparent 
     where locaid = citybid;