Add Online Retail II DuckDB description

online-retail-ii/1.0.0/DATABASE.md

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+This DuckDB contains the **Online Retail II** transaction-line dataset from the UCI
+Machine Learning Repository. The source describes a UK-based non-store retailer, and
+the records cover **1 December 2009 through 9 December 2011**. Unit prices are in GBP.
+
+For ordinary analysis, use `analysis.transaction_lines_nonoverlapping`. It contains
+1,044,848 product lines and removes 22,523 rows where the two source worksheets overlap,
+while intentionally retaining duplicate occurrences within an individual worksheet.
+Each row represents one product line on an invoice, not one complete invoice.
+
+Important analytical conventions:
+
+- `quantity * unit_price_gbp` is the signed line value. Negative quantities therefore
+  reduce net sales rather than counting as positive purchases.
+- Returns and cancellations can have negative quantities, and cancellation invoice IDs
+  commonly begin with `C`.
+- `invoice_ts` is the invoice timestamp and has no timezone attached.
+- `customer_id` and `description` can be missing. A small number of unit prices are zero
+  or negative.
+- `country` is the invoice country. The data contains 43 distinct country values.
+
+The DuckDB also retains both original worksheets in `raw.transaction_lines`. The
+`metadata.dataset` and `metadata.quality_summary` relations contain provenance, row
+counts, and documented quality conditions. Prefer the `analysis` view unless a question
+explicitly concerns the raw overlap or source-sheet provenance.
+
+### Example questions
+
+- How did monthly net sales change during 2011?
+- Which countries outside the United Kingdom generated the most net sales?
+- Which products had the largest cancellation or return quantities?
+- How many customers purchased in consecutive calendar months?
+
+Source: [UCI Online Retail II](https://doi.org/10.24432/C5CG6D). The packaged database
+and this description are distributed under CC BY 4.0.