Index: docs/P3-UseCaseModel/wiki/UseCase0007.md
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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":
+
+{{{
+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 ===
+
+{{{
+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 ===
+
+The Trader picks the crypto by its number from the listed cryptos that are not on the watchlist yet:
+
+{{{
+-- 1. list, numbered, the cryptos not yet on the watchlist
+SELECT c.id, c.symbol, c.name
+  FROM project.crypto c
+ WHERE NOT EXISTS (SELECT 1 FROM project.watchlist_items wi
+                    WHERE wi.watchlist_id = $1 AND wi.crypto_id = c.id)
+ ORDER BY c.symbol;
+
+-- 2. insert the chosen crypto ($2 = its id from the list); do nothing if it's already there
+INSERT INTO project.watchlist_items (watchlist_id, crypto_id)
+VALUES ($1, $2)
+ON CONFLICT (watchlist_id, crypto_id) DO NOTHING;
+}}}
+
+=== Remove a crypto ===
+
+The Trader picks the crypto by its number from the listed cryptos on the watchlist:
+
+{{{
+-- 1. list, numbered, the cryptos on the watchlist
+SELECT c.id, c.symbol, c.name
+  FROM project.watchlist_items wi
+  JOIN project.crypto c ON c.id = wi.crypto_id
+ WHERE wi.watchlist_id = $1
+ ORDER BY c.symbol;
+
+-- 2. delete the chosen crypto ($2 = its id from the list)
+DELETE FROM project.watchlist_items
+ WHERE watchlist_id = $1 AND crypto_id = $2;
+}}}
+
+If the entered number is not one of the listed numbers, system shows "Invalid choice, enter a number from 1 to N." and nothing is changed.