Referencing a calculated field in SQL statement - sql

Question

I have the following schema
CREATE TABLE QUOTE (id int, amount int);
CREATE TABLE QUOTE_LINE (id int, quote_id int, line_amount int);
INSERT INTO QUOTE VALUES(1, 100);
INSERT INTO QUOTE VALUES(2, 200);
INSERT INTO QUOTE VALUES(3, 100);
INSERT INTO QUOTE VALUES(4, 300);
INSERT INTO QUOTE_LINE VALUES(1, 1, 5);
INSERT INTO QUOTE_LINE VALUES(2, 1, 6);
INSERT INTO QUOTE_LINE VALUES(3, 1, 4);
INSERT INTO QUOTE_LINE VALUES(4, 1, 2);
INSERT INTO QUOTE_LINE VALUES(1, 2, 5);
INSERT INTO QUOTE_LINE VALUES(2, 2, 5);
INSERT INTO QUOTE_LINE VALUES(3, 2, 5);
INSERT INTO QUOTE_LINE VALUES(4, 2, 5);
And I need to run the following query:
SELECT QUOTE.id,
line_amount*12 AS amount,
amount*2 as amount_doubled
from QUOTE_LINE
LEFT JOIN QUOTE ON QUOTE_LINE.quote_id=QUOTE.id;
The 3rd line in the query amount*2 as amount_double needs to reference the amount calculated in the prior line i.e. line_amount*12 AS amount.
However if I run this query, it picks the amount from the QUOTE table instead the amount that was calculated. How can I make my query use the calculated amount without changing the name of the calculated field?
Here is the sqlfiddle for this:
http://sqlfiddle.com/#!17/914b2/1
Note: I understand that I can create a sub-query, CTE or a lateral join, but the tables I am working are very very wide tables, and the queries have many many joins. As such, I need to keep the LEFT INNER JOINS and also I don't always know if a calculated field will be duplicated in JOINed table or not. Table structures change.

Answer 1

Move the definition to the FROM clause using a LATERAL JOIN:
select q.id, v.amount, v.amount * 2 as as amount_doubled
from QUOTE_LINE ql left join
QUOTE q
on ql.quote_id = q.id CROSS JOIN LATERAL
(values (line_amount*12)) v(amount);
You can also use a subquery or CTE, but I like the lateral join method.
Note: I would expect QUOTE to be the first table in the LEFT JOIN.

Answer 2

Qualify all column names with the table name and use a subquery:
SELECT q.id,
q.amount,
q.amount * 2 AS amount_doubled
FROM (SELECT quote.id,
quote_line.line_amount * 12 AS amount,
FROM quote_line
LEFT JOIN quite
ON quote_line.quote_id = quote.id
) AS q;

Answer 3

Just a little simple algebra resolves the issue quite easily. It is clear that calculated amount is 12 times the line_amount and that amount_doubles is 2 times that. So
select q.id
, ql.line_amount*12 as amount
, ql.line_amount*12*2 as amount_doubled
from quote_line ql
left join quote q
on ql.quote_id = q.id;
However, child left join parent seems strange as it basically says "Give me the quote line amounts where there is no quote". One would hope a FK from line to quote would prevent that from happening.
If so then a inner join would suffice. Further if the id is the only column from quote the join can removed by taking quote_id from quote_line. So perhaps reducing to:
select ql.quote_id as id
, ql.line_amount*12 as amount
, ql.line_amount*24 as amount_doubled
from quote_line ql;