MySQL-从嵌套子查询引用外部字段

Question

How is possible to reference an external field from a nested subquery?

I'll explain better very fastly with an example:

SELECT
x
, (SELECT t1.x) as x1
/*, (SELECT x FROM (SELECT t1.x) as t2) as x2*/
FROM
(SELECT 1 as x UNION SELECT 2 as x UNION SELECT 3 as x) as t1;

If I uncomment the commented subquery, I get a "Unknown table 't1' in field list" error but I need to refer for a complex calculation to that variable from a 2-level nested subquery (it's not possible to do with a Join).

Answer 1

This is not possible according to MySQL manual . You can try using VIEWs instead of derived tables or list all your derived tables in the outermost FROM clause

Answer 2

You can't use table alias in subquery, because it is out of scope. You should (eventually) recode the subquery like this:

SELECT  
    x,
    t1.x  as x1,
    (SELECT x 
     FROM (SELECT 1 as x 
           UNION 
           SELECT 2 as x 
           UNION SELECT 3 as x) as t2) as x2
FROM 
    (SELECT 1 as x 
     UNION 
     SELECT 2 as x 
     UNION 
     SELECT 3 as x) as t1;

or create a proper view and use the view for access to the data

create view my_view as 

 SELECT 1 as x 
 UNION 
 SELECT 2 
 UNION 
 SELECT 3  
 ;

SELECT x
,  t1.x  as x1
, (SELECT x FROM  my_view  as t2) as x2
FROM my_view as t1
;