Index: P3-UseCaseModel/UseCase0004.md
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+# Use-case 0004 — Place market BUY order
+
+**Initiating actor:** Trader
+
+**Other actors:** Market Simulator (indirect — supplies the current price via `market_trades`).
+
+A Trader buys a crypto asset at the current market price. The operation touches five tables (`orders`, `users`, `holdings`, `transactions`, `market_trades`) and must either all succeed or all roll back.
+
+## Scenario
+
+1. Trader chooses "Place market BUY order".
+2. System lists the available markets with their latest price:
+
+   ```sql
+   SELECT m.id, c.symbol, m.quote_currency, COALESCE(lp.price, 0)
+     FROM project.markets m
+     JOIN project.crypto  c  ON c.id = m.crypto_id
+     LEFT JOIN project.v_latest_prices lp ON lp.market_id = m.id
+    WHERE m.is_active = true
+    ORDER BY c.symbol;
+   ```
+3. Trader enters a market symbol, e.g. `ETH`.
+4. System resolves the market and looks up the latest price:
+
+   ```sql
+   SELECT m.id, c.id AS crypto_id, c.symbol, m.quote_currency
+     FROM project.markets m
+     JOIN project.crypto c ON c.id = m.crypto_id
+    WHERE upper(c.symbol) = upper($1) AND m.is_active = true;
+
+   SELECT price FROM project.v_latest_prices WHERE market_id = $2;
+   ```
+5. Trader enters a quantity.
+6. System computes notional = quantity × price, opens a transaction, and does:
+
+   ```sql
+   BEGIN;
+
+   INSERT INTO project.orders
+       (user_id, market_id, side, type, status, quantity, price, executed_at)
+   VALUES
+       ($user_id, $market_id, 'buy', 'market', 'executed', $qty, $price, now())
+   RETURNING id;  -- captured as $order_id
+
+   SELECT available_balance FROM project.users WHERE id = $user_id FOR UPDATE;
+   -- abort if available_balance < notional
+
+   UPDATE project.users
+      SET available_balance = available_balance - $notional,
+          invested_balance  = invested_balance  + $notional,
+          updated_at        = now()
+    WHERE id = $user_id;
+
+   -- Upsert holding with running weighted-average price:
+   SELECT quantity, avg_price
+     FROM project.holdings
+    WHERE user_id = $user_id AND crypto_id = $crypto_id
+    FOR UPDATE;
+
+   -- Either INSERT (new holding) or UPDATE (existing), computing
+   -- new_avg = (old_qty*old_avg + $qty*$price) / (old_qty + $qty)
+
+   INSERT INTO project.transactions
+       (user_id, type, amount, currency, related_order, description)
+   VALUES
+       ($user_id, 'buy', -$notional, 'USD', $order_id, 'Market buy ...');
+
+   INSERT INTO project.market_trades
+       (market_id, executed_at, price, quantity, side, source)
+   VALUES
+       ($market_id, now(), $price, $qty, 'buy', 'user');
+
+   COMMIT;
+   ```
+7. System confirms: `Order executed: buy 0.0100 BTC @ 67140.000000 (notional 671.4000 USD)`.
+
+### Alternate flow 6a — insufficient funds
+
+If `available_balance < notional`, the entire transaction rolls back and system shows "Insufficient funds: need X, have Y."
+
+### Alternate flow 4a — market not found
+
+If the entered symbol does not match any active market, system shows "market X not found" and returns to the authenticated menu without opening a transaction.