Archive Database Step 4: Count the Gaps

Write one query reporting, for every column that can be missing, how many rows are missing it, using the difference between COUNT(*) and COUNT(column). Then write a sentence for each saying what a missing value means there: unknown, not applicable, or not yet recorded. Those are three different things and the query cannot tell them apart. Finally, decide for each whether you would COALESCE it in an analysis, and write down why.