Validating an SQL query - sql

Question

I am a beginner with SQL and looking to write a query to identify users whose first transaction was a successful card payment over an equivalent value of 10 USD (amounts are in different currencies).
This is a theoretical exercise whereby I have the databases in excel but currently cannot access any SQL servers to validate this query.
Firstly I have defined the result set as follows:
SELECT t.user_id, min(t.created_date), (t.amount / fx.rate / Power (10, cd.exponent) AS amount)
FROM transactions AS t
This should yield the user ID, earliest date of transaction and the transaction amount in USD (original transaction converted into USD and converted into a cash amount from an integer amount).
Fairly comfortable with the last formula, just want to make sure the referencing below brings the fx.rate and cd.exponent correctly so it can actually run:
JOIN fx_rates AS fx
ON ( fx.ccy = t.currency
AND fx.base_ccy = 'USD' )
JOIN currency_details AS cd
ON cd.currency = t.currency
The above should ensure the 'amount' column has all the references necessary to be calculated.
Finally I am looking to apply a set of restrictions so data includes only completed card payments over 10 USD:
WHERE t.type='card_payment'
AND t.state='completed'
AND amount>=10
This is the tricky bit as I read that you can't reference an alias ('amount') as it isn't really in the result set but not sure if that applies here.
I have two questions:
1) Would this query produce a list of first transactions which were over 10USD? I don't want it to find when/if the transaction reached that threshold. I am only interested in the first transaction for each user. If the answer is no, would I be better of creating a table with first transactions and filtering on that instead? I honestly thought that's what I'm doing here.
2) Is referencing the alias 'amount' allowed within the query? If not, is another SELECT required here?
Full query
SELECT t.user_id, min(t.created_date), (t.amount / fx.rate / Power (10, cd.exponent) AS amount)
FROM transactions AS t
JOIN fx_rates AS fx
ON ( fx.ccy = t.currency
AND fx.base_ccy = 'USD' )
JOIN currency_details AS cd
ON cd.currency = t.currency
WHERE t.type='card_payment'
AND t.state='completed'
AND amount>=10
-------UPDATE 1-------
Following numerous comments and answers the updated query is as follows:
SELECT t.user_id, t.created_date,
  (t.amount / fx.rate / Power(10, cd.exponent)) AS amount
FROM (
    SELECT *, Row_Number () OVER
  (PARTITION BY t.user_id ORDER BY t.created_date) AS RowNum
    FROM transactions AS t)
  JOIN fx_rates fx 
    ON ( fx.ccy = t.currency
      AND fx.base_ccy = ‘USD') 
  JOIN currency_details cd 
    ON cd.currency = t.currency
WHERE RowNum = 1 
  AND t.type = ‘card_payment‘
  AND t.state = ‘completed‘
  AND (t.amount / fx.rate / Power(10, cd.exponent)) >= 10
GROUP  BY t.user_id;

Answer 1

The amount column name alias is not available in where condition.
You must repeat the code
SELECT t.user_id, min(t.created_date), (t.amount / fx.rate / Power (10, cd.exponent) ) AS amount
FROM transactions AS t
JOIN fx_rates AS fx
ON ( fx.ccy = t.currency
AND fx.base_ccy = 'USD' )
JOIN currency_details AS cd
ON cd.currency = t.currency
WHERE t.type='card_payment'
AND t.state='completed'
AND (t.amount / fx.rate / Power (10, cd.exponent) )>=10
this because the where clause is evaluated before the select clause so .. in where clause the alias column name is not available .. is used the original (row related) amount column content

Answer 2

"Would this query produce a list of first transactions which were over 10USD?"
No. Besides the fact that what you've written will produce multiple errors, the logic you are attempting here will not limit the results to the first ever transaction, rather it would return the first date of a completed card payment over $10.
"Is referencing the alias 'amount' allowed within the query?"
No. But you can repeat the same part of the select statement.
below is all untested
There are a variety of ways to accomplish what you want. One of the easiest to understand (I think) is to start with a query that orders the transactions by date per user:
select *, row_number() over (partition by user_id order by created_date) rown
from transactions
rown in the above query is the row number for each user (partition by user_id) with the earliest date (order by created_date) being row 1. The output of this query will have multiple rown columns being 1. Each of those rows represents the first ever transaction for that user. You can throw that query into a subquery and select only the rows where rown = 1
select *
from (
select *, row_number() over (partition by user_id order by created_date) rown
from transactions
) t
where t.rown=1
NOW you can add the rest of your stuff (note I don't know what to do with your power() statement)
select t.user_id, t.created_date, t.amount / fx.rate / 10.0 / cd.exponent AS amount
from (
select *, row_number() over (partition by user_id order by created_date) rown
from transactions
) t
inner JOIN fx_rates AS fx
ON fx.ccy = t.currency
AND fx.base_ccy = 'USD'
inner JOIN currency_details AS cd ON cd.currency = t.currency
where t.rown=1
and t.type='card_payment'
AND t.state='completed'
and t.amount / fx.rate / 10.0 / cd.exponent > 10.0
You might want >= 10.0 if you actually want at least $10.

Answer 3

You can also use CTE(Common Table Expressions)
WITH User_CTE (user_id , created_date , amount, RowNum)
AS
(
SELECT user_id , t.created_date , (t.amount / fx.rate / Power(10, cd.exponent)) AS amount
,ROW_NUMBER over (PARTITION BY t.user_id ORDER BY t.created_date asc ) AS RowNum
FROM transactions t
JOIN fx_rates fx
ON ( fx.ccy = t.currency
AND fx.base_ccy = 'USD')
JOIN currency_details cd
ON cd.currency = t.currency
WHERE
t.type = 'card_payment'
AND t.state = 'completed'
AND (t.amount / fx.rate / Power(10, cd.exponent)) >= 10
)
SELECT
user_id
, created_date
, amount
FROM
User_CTE
WHERE
RowNum =1