Select the date of a UserIDs first/most recent purchase - sql

Question

I am working with Google Analytics data in BigQuery, looking to aggregate the date of last visit and first visit up to UserID level, however my code is currently returning the max visit date for that user, so long as they have purchased within the selected date range, because I am using MAX().
If I remove MAX() I have to GROUP by DATE, which I don't want as this then returns multiple rows per UserID.
Here is my code which returns a series of dates per user - last_visit_date is currently working, as it's the only date that can simply look at the last date of user activity. Any advice on how I can get last_ord_date to select the date on which the order actually occurred?
SELECT
customDimension.value AS UserID,
# Last order date
IF(COUNT(DISTINCT hits.transaction.transactionId) > 0,
(MAX(DATE)),
"unknown") AS last_ord_date,
# first visit date
IF(SUM(totals.newvisits) IS NOT NULL,
(MAX(DATE)),
"unknown") AS first_visit_date,
# last visit date
MAX(DATE) AS last_visit_date,
# first order date
IF(COUNT(DISTINCT hits.transaction.transactionId) > 0,
(MIN(DATE)),
"unknown") AS first_ord_date
FROM
`XXX.XXX.ga_sessions_20*` AS t
CROSS JOIN
UNNEST (hits) AS hits
CROSS JOIN
UNNEST(t.customdimensions) AS customDimension
CROSS JOIN
UNNEST(hits.product) AS hits_product
WHERE
parse_DATE('%y%m%d',
_table_suffix) BETWEEN DATE_SUB(CURRENT_DATE(), INTERVAL 30 day)
AND DATE_SUB(CURRENT_DATE(), INTERVAL 1 day)
AND customDimension.index = 2
AND customDimension.value NOT LIKE "true"
AND customDimension.value NOT LIKE "false"
AND customDimension.value NOT LIKE "undefined"
AND customDimension.value IS NOT NULL
GROUP BY
UserID

Answer 1

the most efficient and clear way to do this (and also most portable) is to have a simple table/view that has two columns: userid, last_purchase and another that has other two cols userid, first_visit.
then you inner join it with the original raw table on userid and hit timestamp to get, say, the session IDs you're interested in. 3 steps but simple, readable and easy to maintain
It's very easy to hit too much complexity for a query that relies on first or last purchase/action (just look at the unnest operations you have there) that is becomes unusable and you'll spend way too much time trying to figure out the meaning of the output.
Also keep in mind that using the wildcard in the query has a limit of 1000 tables, so your last and first visits are in a rolling window of 1000 days.