Index: P3-UseCaseModel/UseCase0007.md
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--- P3-UseCaseModel/UseCase0007.md	(revision d8ce4e224bde01e8f87558691226298db515617e)
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+# Use-case 0007 — Manage watchlist
+
+**Initiating actor:** Trader
+
+**Other actors:** —
+
+A Trader keeps a list of crypto assets they want to monitor. Adding an asset that is already on the list is a no-op (idempotent).
+
+## Scenario
+
+1. Trader chooses "Manage watchlist".
+2. System ensures the Trader has a default watchlist named "Favorites":
+
+   ```sql
+   SELECT id FROM project.watchlists
+    WHERE user_id = $1
+    ORDER BY created_at LIMIT 1;
+
+   -- if no row:
+   INSERT INTO project.watchlists (user_id, name)
+   VALUES ($1, 'Favorites')
+   RETURNING id;
+   ```
+3. System shows the sub-menu: List / Add / Remove / Back.
+
+### List items
+
+```sql
+SELECT c.symbol, c.name, COALESCE(lp.price, 0)
+  FROM project.watchlist_items wi
+  JOIN project.crypto c ON c.id = wi.crypto_id
+  LEFT JOIN project.markets m
+         ON m.crypto_id = c.id AND m.quote_currency = 'USD'
+  LEFT JOIN project.v_latest_prices lp
+         ON lp.market_id = m.id
+ WHERE wi.watchlist_id = $1
+ ORDER BY c.symbol;
+```
+
+### Add a crypto
+
+```sql
+-- 1. resolve the symbol to a crypto_id
+SELECT id FROM project.crypto WHERE upper(symbol) = upper($1);
+
+-- 2. insert the item; do nothing if it's already there
+INSERT INTO project.watchlist_items (watchlist_id, crypto_id)
+VALUES ($watchlist_id, $crypto_id)
+ON CONFLICT (watchlist_id, crypto_id) DO NOTHING;
+```
+
+### Remove a crypto
+
+```sql
+DELETE FROM project.watchlist_items
+ WHERE watchlist_id = $1
+   AND crypto_id = (
+       SELECT id FROM project.crypto
+        WHERE upper(symbol) = upper($2)
+   );
+```
+
+If the delete affects zero rows, system shows "Not in watchlist."