MySQL的，在同一查询中设置一个结果值到另一个值

Question

我有以下SQL语句：

SELECT `entity_id`,`parent_id`,`name` 
    FROM category 
    WHERE is_active = 1 
    ODER BY parent_id asc, position asc; 

它返回类似下面的结果：

+-----------+-----------+------------+----------+ 
| id  | parent_id | Name  | desc  | 
+-----------+-----------+------------+----------+ 
| 1   | NULL  | Fruit  | NULL  | 
+-----------+-----------+------------+----------+ 
| 2   | 1   | Apple  | NULL  | 
+-----------+-----------+------------+----------+ 

我想更新查询，以便在父ID字段中返回相应的名称。所以，我的输出会是这样：

+-----------+-----------+------------+----------+ 
| id  | parent_id | Name  | desc  | 
+-----------+-----------+------------+----------+ 
| 1   | NULL  | Fruit  | NULL  | 
+-----------+-----------+------------+----------+ 
| 2   | Fruit  | Apple  | NULL  | 
+-----------+-----------+------------+----------+ 

我一直在使用同一个表内连接玩这个查询了一段时间，虽然我不知道这是正确的做法。任何人都可以推荐一种方法。

Answer 1

这必须是这个，你在寻找：

SELECT c.`entity_id`,c. p.`name` as parent_id, c.`name` 
    FROM category c 
    LEFT JOIN category p WHERE c.`parent_id = p.id 
    WHERE c.is_active = 1 
    ORDER BY c.parent_id ASC, c.position ASC; 

Answer 2

贝恩德Buffen答案是正确的，另一种方式来做到这一点会使用主查询

SELECT c.`entity_id`, IFNULL((SELECT c2.`name` FROM category c2 WHERE c.`entity_id` = c2.`parent_id`), '') as parent_name, c.`name` 
FROM category c 
WHERE c.is_active = 1 
ORDER BY c.parent_id asc, c.position asc;