How to save the results of a select expression as a variable? - sql

Question

In the following Postgresql sql, is there a way to save mo.delivered_at - mo.created_at as a variable so I don't have to repeat myself?
SELECT
to_char(mo.created_at,'MM-YYYY') AS month,
mo.sku_key as sku,
c.name,
COUNT(*) as total,
COUNT(*) FILTER (WHERE mo.delivered_at - mo.created_at < interval '3 days') as three_days,
COUNT(*) FILTER (WHERE mo.delivered_at - mo.created_at > interval '3 days' and mo.delivered_at - mo.created_at <= interval '6 days') as six_days,
COUNT(*) FILTER (WHERE mo.delivered_at - mo.created_at > interval '6 days' and mo.delivered_at - mo.created_at <= interval '9 days') as nine_days,
COUNT(*) FILTER (WHERE mo.delivered_at - mo.created_at > interval '9 days') as ten_days,
min(mo.delivered_at - mo.created_at),
max(mo.delivered_at - mo.created_at),
percentile_disc(0.5) within group (order by mo.delivered_at - mo.created_at) as median,
avg(mo.delivered_at - mo.created_at) as average
FROM medication_order mo
LEFT JOIN subscription s ON s.id=mo.subscription_id
LEFT JOIN condition c on s.condition_id = c.id
WHERE
mo.status = 'DELIVERED' AND
mo.payment_preference = 'INSURANCE' AND
mo.created_at > '2020-01-01' AND
mo.delivered_at IS NOT null AND
mo.sku_key != 'manual_order_sku'
GROUP BY month, mo.sku_key, c.name

Answer 1

You can compute the derived value in a subquery or CTE as has been suggested.
But there is more. This should be faster (and correct). And can be sorted properly, too:
SELECT
to_char(mo.month,'MM-YYYY') AS month, -- optionally prettify
mo.sku,
s.condition_id, -- I added this to make the result unambiguous
(SELECT name FROM condition WHERE id = s.condition_id) AS condition_name,
COUNT(*) AS total,
COUNT(*) FILTER (WHERE mo.my_interval < interval '3 days') AS three_days,
COUNT(*) FILTER (WHERE mo.my_interval > interval '3 days' AND mo.my_interval <= interval '6 days') AS six_days,
COUNT(*) FILTER (WHERE mo.my_interval > interval '6 days' AND mo.my_interval <= interval '9 days') AS nine_days,
COUNT(*) FILTER (WHERE mo.my_interval > interval '9 days') AS ten_days,
min(mo.my_interval),
max(mo.my_interval),
percentile_disc(0.5) WITHIN GROUP (ORDER BY mo.my_interval) AS median,
avg(mo.my_interval) AS average
FROM (
SELECT
date_trunc('month', mo.created_at) AS month, -- faster, keeps ORDER
delivered_at - created_at AS my_interval, -- your core request
sku_key AS sku
FROM medication_order mo
WHERE status = 'DELIVERED' -- filter early
AND payment_preference = 'INSURANCE'
AND created_at > '2020-01-01'
AND delivered_at IS NOT NULL
AND sku_key <> 'manual_order_sku'
) mo
LEFT JOIN subscription s ON s.id = mo.subscription_id
GROUP BY mo.month, mo.sku, s.condition_id -- GROUP BY unique ID! Correct - and cheaper, too
ORDER BY mo.month, mo.sku, s.condition_id; -- my addition: sorting by date works across years, 'MM-YYYY' does not
Aside: condition.name should probably be UNIQUE. And "name" is almost never a good name.

Answer 2

You could just compute the information in a subquery when selecting from the table:
SELECT
to_char(mo.created_at,'MM-YYYY') AS month,
mo.sku_key as sku,
c.name,
COUNT(*) as total,
COUNT(*) FILTER (WHERE mo.delivery_interval < interval '3 days') as three_days,
COUNT(*) FILTER (WHERE mo.delivery_interval > interval '3 days' and mo.delivery_interval <= interval '6 days') as six_days,
COUNT(*) FILTER (WHERE mo.delivery_interval > interval '6 days' and mo.delivery_interval <= interval '9 days') as nine_days,
COUNT(*) FILTER (WHERE mo.delivery_interval > interval '9 days') as ten_days,
min(mo.delivery_interval),
max(mo.delivery_interval),
percentile_disc(0.5) within group (order by mo.delivery_interval) as median,
avg(mo.delivery_interval) as average
FROM (
SELECT mo.*, mo.delivery_interval delivery_interval --> here
FROM medication_order
) mo
LEFT JOIN subscription s ON s.id=mo.subscription_id
LEFT JOIN condition c on s.condition_id = c.id
WHERE
mo.status = 'DELIVERED' AND
mo.payment_preference = 'INSURANCE' AND
mo.created_at > '2020-01-01' AND
mo.delivered_at IS NOT null AND
mo.sku_key != 'manual_order_sku'
GROUP BY month, mo.sku_key, c.name