MySQL语法错误，关于union和as关键字

Question

select distinct id, item from 
(
    (
        (
            select ID as id, "INNER" as item 
            from TREE 
            where ID in 
            (
                select P_ID from TREE as T1
            )  
            and P_ID is not null
        ) 
        as T3
    )
    union
    (
        (
            select ID as id, "ROOT" as item 
            from TREE 
            where P_ID IS NULL
        ) 
        as T2
    )
)

TREE 表的结构如下:

ID, P_ID
1,2
3,4
5,6

我是个SQL小白，真不知道为什么执行的时候总是出问题。

Error Code: 1064. You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'as T2 ) )' at line 22

你能告诉我吗？谢谢!

Answer 1

不知道您要通过查询实现什么，但可以修复语法错误，您可以将查询重写为

SELECT DISTINCT 
  id,
  item 
FROM(
  SELECT 
    ID AS id,
    "INNER" AS item 
  FROM
    TREE 
  WHERE ID IN (SELECT P_ID FROM TREE) 
  AND P_ID IS NOT NULL 

  UNION

  SELECT 
    ID AS id,
    "ROOT" AS item 
  FROM
    TREE 
  WHERE P_ID IS NULL
) as t