**Performance Tips:**

- ROLLUP/CUBE more efficient than UNION of multiple GROUP BY

- Can be expensive on large datasets

- Index grouping columns

- Consider materialized views for frequent use

ROLLUP/CUBE more efficient than multiple GROUP BY with UNION

Can be expensive on large datasets

Index grouping columns

Consider materialized views for frequent reports

Test performance with production data volumes

Confusing NULL subtotals with NULL data

Not using GROUPING() to identify subtotal rows

Using CUBE when ROLLUP sufficient (unnecessary rows)

Forgetting NULLS LAST in ORDER BY

Not labeling subtotal rows for readability