How to use alias in a left join?

问题

I have a problem in my MySQL command

select upper(file_type) as 'ItemType', ponum as 'Policy Number', office as 'Office', fullname as 'FullName', remarks as 'Remarks', concat(x.id, '-',x.file_type) as 'Identification'
from (select * from active_tb
union
select * from processed_tb) as x
left join filelocation as y on x.identification = y.f_id

I get unknown column x.Identification when trying to execute the query.

First I join two different tables, then select the columns I need, then assign an alias. I need to concatenate the identification column.

But I cannot use the alias in a left join.

回答1:

Just build your identification column in your left join sub query first.

SELECT upper(file_type) AS 'ItemType',
       ponum            AS 'Policy Number',
       office           AS 'Office',
       fullname         AS 'FullName',
       remarks          AS 'Remarks',
       x.identification
FROM   (SELECT *,
               concat(x.id, '-', x.file_type) AS 'identification'
        FROM   active_tb
        UNION
        SELECT *,
               concat(x.id, '-', x.file_type) AS 'identification'
        FROM   processed_tb) AS x
       LEFT JOIN filelocation AS y
              ON x.identification = y.f_id

回答2:

I solved it now. Here is my code:

select ItemType, 'Policy Number', Office, fullname, identification, f_location
from (select upper(file_type) as 'ItemType', ponum as 'Policy Number', office as 'Office', fullname as 'FullName', remarks as 'Remarks', concat(id, '-',file_type) as 'Identification' from active_tb
union
select upper(file_type) as 'ItemType', ponum as 'Policy Number', office as 'Office', fullname as 'FullName', remarks as 'Remarks', concat(id, '-',file_type) as 'Identification' from processed_tb) as x
left join filelocation on filelocation.f_id = x.Identification

Please comment other suggestions.

来源：https://stackoverflow.com/questions/42848510/how-to-use-alias-in-a-left-join

标签

mysql

left-join

alias