Select specific columns by their alias, and later order by it - sql

Question

My table are the following
+----+----------+--------+
| id | priority | User |
+----+----------+--------+
| 1 | 2 | [null] |
| 2 | 1 | [null] |
| 3 | 3 | Tony |
| 4 | 2 | John |
| 5 | 2 | Andy |
| 6 | 1 | Mike |
+----+----------+--------+
My goal is to extract them, and order by the following combined conditions:
priority = 1
User is null
+----+----------+--------+-----------+
| id | priority | User | peak_rows |
+----+----------+--------+-----------+
| 1 | 2 | [null] | 1 |
| 2 | 1 | [null] | 1 |
| 6 | 1 | Mike | 0 |
| 3 | 3 | Tony | 1 |
| 4 | 2 | John | 0 |
| 5 | 2 | Andy | 0 |
+----+----------+--------+-----------+
This is what I guess I can do
select
id,
CASE WHEN priority = 1 THEN 1 ELSE 0 END as c1,
CASE WHEN User is NULL THEN 1 ELSE 0 END as c2,
c1 + c2 AS peak_rows
FROM mytable
ORDER BY peak_rows DESC
but it cause an error:
ERROR: column "c1" does not exist
LINE 5: c1+ c2as pp
^
SQL state: 42703
Character: 129
I don't know why I make 2 columns(c1 and c2), but I can not use it later.
Any good idea to do that?

Answer 1

You are not making two columns and using them later, you are making them and want to use them at the same time. You could use a subquery.
SELECT a.id, a.priority, a.User, a.c1 + a.c2 AS peak_rows
FROM
(SELECT id,
priority,
User,
CASE WHEN priority = 1 THEN 1 ELSE 0 END as c1,
CASE WHEN User IS NULL THEN 1 ELSE 0 END as c2,
FROM mytable) a
ORDER BY peak_rows DESC;

Answer 2

select
id,
CASE WHEN priority = 1 THEN 1 ELSE 0 END as c1,
CASE WHEN User is NULL THEN 1 ELSE 0 END as c2,
(CASE WHEN priority = 1 THEN 1 ELSE 0) + ( CASE WHEN User is NULL THEN 1 ELSE 0 END) AS peak_rows
FROM mytable
ORDER BY peak_rows DESC

Answer 3

I suppose your aim is to order by those c1 and c2, so you can directly use in the order by clause. You just need to interchange 0 and 1 in the case..when statements. And depending on your priority=1 criteria id=2 must stay at the top.
with mytable( id, priority, "User" ) as
(
select 1 , 2, null union all
select 2, 1, null union all
select 3, 3, 'Tony' union all
select 4, 2, 'John' union all
select 5, 2, 'Andy' union all
select 6, 1, 'Mike'
)
select *
from mytable
order by ( case when priority = 1 then 0 else 1 end ) +
( case when "User" is null then 0 else 1 end );
id priority User
-- -------- -------
2 1 [null]
1 2 [null]
6 1 Mike
3 3 Tony
4 2 John
5 2 Andy
Demo