Use same alias for union query SQL - sql

Question

So I came across a problem/question yesterday.
I am building a chat (with AJAX) and use two tables:
TABLE users -> 'name', 'username', 'password', 'time'
TABLE messages -> 'sendFrom', 'sendTo', 'message', 'time'
So an example message now would be
'foo' | 'bar' | 'Hey, how are you?' | 130611134427611
I was told the correct way to do this is, instead, to use an ID column, and use that as a Primary Key instead of the username (which, anyway, makes sense).
OK, so now this looks like
TABLE users -> 'ID', 'name', 'username', 'password', 'time'
TABLE messages -> 'sendFrom', 'sendTo', 'message', 'time'
So an example message now would be
'22' | '7' | 'Hey, how are you?' | 130611134427611
I've managed to JOIN both tables to return the rows as on the first example message, but since I am detecting user keypresses too, I need to scan the table twice, so:
SELECT *
FROM (SELECT *
FROM (SELECT *
FROM messages
WHERE sendTo = '$username'
AND time > (SELECT time FROM users
WHERE username = '$username' LIMIT 1)
AND message <> '$keypressCode'
ORDER BY time DESC LIMIT 30)
ORDER BY time ASC)
UNION
SELECT *
FROM (SELECT *
FROM messages
WHERE message = '$keypressCode'
AND time > (SELECT time FROM users
WHERE username = '$username' LIMIT 1)
AND sendTo = '$username' LIMIT 1);
But now, of course, I don't just select from messages; instead, I use a long query like
SELECT * FROM (
SELECT u1.ID as sendTo, u2.ID as sendFrom, messages.message, .....
.....
.....
.....
.....
) as messages;
that MUST BE INSERTED just in the place of messages (I haven't tried this yet, but I think is like that. See, the thing is I DuckDuckGo'ed and Googled and found nothing, so I came here)
My first question is:
Is there a way to use ALIAS for the table messages so I don't have to scan it TWICE? So, instead, I just save the above query using ALIAS as a table called messages and select data from it twice, once in each part of UNION.
In addition, the answer to the first question would also be an answer for:
Is there a way to use ALIAS to save the time selected from the table? (since, again, I am searching for it TWICE).
In practice, what I am doing may not be unefficient (since there will be at most 20 users), but what if?
Also, I am a mathematician, and like it or not, I like to worry a lot about efficiency!
Thank you so much in advance, I hope I made myself clear.

Answer 1

I am not sure but it does look as if you want a view.
Define that query like this:
CREATE VIEW MyMessageView
AS
SELECT ...
FROM ...
...
Now you can use that view in any context where an ordinary table can be used: in a FROM clause, in a JOIN clause, as a subquery etc.:
SELECT ...
FROM MyMessageView
WHERE ...
...
UNION
SELECT ...
FROM MyMessageView
WHERE ...

Answer 2

Instead of using UNION, put an OR in the condition:
SELECT *
FROM messages
WHERE time > (SELECT time FROM users
WHERE username = '$username' LIMIT 1)
AND (message <> '$keypressCode' AND sendTo = '$username'
OR message = '$keypressCode')

Answer 3

I am answering my own question, since I consider what people might be looking for is a VIEW.
First, define that query like this:
CREATE VIEW MyViewTable
AS
SELECT ...
FROM ...
...
...;
Now you can use that view (which is a sepparate query) in any context where an ordinary table can be used: in a FROM clause, in a JOIN clause, as a subquery etc.:
SELECT ...
FROM MyViewTable
WHERE ...
...
UNION
SELECT ...
FROM MyViewTable
WHERE ...
but with a few restrictions:
You cannot SELECT from your view using subqueries, such as
SELECT * FROM MyViewTable WHERE someColumn = (SELECT ... ...)
but (as normal) you can use subqueries when creating the VIEW and in the main query.
The SELECT statement cannot refer to prepared statement parameters.
The definition cannot refer to a TEMPORARY table, and you cannot create a TEMPORARY view.
(there are more, but this are, in my opinion, among the most common queries, so the restrictions might be among the most common errors. See SQLite reference for more information. ).