Multi-table JOIN with GROUP BY on two dimensions
Building the 2025 monthly spend trend by department. OpEx only, posted only, all 11 months we have so far. Columns: `department`, `month`, `total_spend`. Sort by department then month.
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Three-table JOIN: `fact_transactions` + `dim_accounts` (for OpEx filter) + `dim_cost_centers` (for department).
Filter `LOWER(account_type) = 'opex'` — there's no casing trap here since the miscase is on Revenue, but the habit matters.
GROUP BY department, month — use `EXTRACT(MONTH FROM txn_date::date)` for the month number.