如何使用列别名在 SQL 中使用 UNION 排序？

Question

Here is my very simple MYSQL request:

(SELECT start, name, id, info FROM `table1`)
UNION
(SELECT end, name, id, info FROM `table1`)

I want to sort the result by 1st column and I guessed I need to use aliases:

(SELECT start as mydate, name, id, info FROM `table1`)
UNION
(SELECT end as mydate, name, id, info FROM `table1`)
ORDER BY mydate

I was surprised that MYSQL threw this error:

 "Unknown column 'mydate' in 'where clause'" 

I ended with this working bad practice:

(SELECT start, name, id, info FROM table1)
UNION
(SELECT end, name, id, info FROM table1)
ORDER BY 1

source: https://www.mysqltutorial.org/sql-union-mysql.aspx

But I would like to understand my error !

Answer 1

Use a nested query.

SELECT q.* 
  FROM (
              SELECT start as mydate, name, id, info FROM `table1`
              UNION ALL
              SELECT end as mydate, name, id, info FROM `table1`
       ) AS q
 ORDER BY q.mydate

The inner query builds up your result set and the outer one orders it. The MySQL query planner is reasonably smart about optimizing this sort of thing.

By the way UNION removes duplicates and UNION ALL does not.