What device actually contains in ab-test-checkout

Working through ab-test-checkout again. device is the column people trip over, so here is what it actually looks like.

6,000 rows, no nulls, 3 distinct values. desktop, mobile, or tablet.

3 values, and they are not evenly spread: mobile 54.7%, desktop 38.5%, tablet 6.8%. The largest takes 54.7% on its own.

Read the distinct list rather than the distinct count. Casing differences and trailing whitespace produce values that look identical in a report and group separately in SQL, and the count will not show you that — select distinct device order by 1 will, in about a second.

select
  count(*)                 as rows,
  count(device)            as present,
  count(*) - count(device) as nulls,
  count(distinct device)   as distinct_values
from ab_test_checkout;

Low cardinality, stable values — this is a column you can group by without thinking about it, and a reasonable candidate for a chart facet. Check the distinct list, not just the count, because a stray casing variant hides in the count and shows up in the group-by.

Where this bites: a group-by on this column produces 3 rows today. If it is a column an upstream system can add values to, it produces an unknown number tomorrow, and any dashboard laid out for 3 categories reflows without warning. Values arriving is a schema change that no schema check catches.

The grain is one row per exposed user, which is the context every one of those numbers depends on. None of them survive a change of grain, which is why "profile the column" and "profile the table" are the same job. Full schema, and the CSV, on the dataset page.