TEXTJOIN to STRING_AGG

TEXTJOIN down a column is STRING_AGG. ignore_empty is WHERE col IS NOT NULL AND col <> '.

SELECT customer_id, STRING_AGG(sku, ', ') AS skus
FROM OrderLines
WHERE sku IS NOT NULL AND sku <> ''
GROUP BY customer_id;

STRING_AGG needs 2017+. Older servers: the FOR XML PATH trick.

Canonical translation

SELECT customer_id, STRING_AGG(sku, ', ') AS skus
FROM OrderLines
WHERE sku IS NOT NULL AND sku <> ''
GROUP BY customer_id;