feat: 演示页新增事务/标量子查询/外键级联/高级条件 4组预设

site/demo.html

@@ -113,6 +113,10 @@ SELECT COUNT(*) as total_users, AVG(age) as avg_age FROM users;</textarea>
         <button class="btn btn-preset" onclick="loadPreset('ddl')">🏗 DDL</button>
         <button class="btn btn-preset" onclick="loadPreset('crud')">✏️ CRUD</button>
         <button class="btn btn-preset" onclick="loadPreset('subquery')">🔍 子查询</button>
+        <button class="btn btn-preset" onclick="loadPreset('tx')">🔒 事务</button>
+        <button class="btn btn-preset" onclick="loadPreset('scalar')">🎯 标量子查询</button>
+        <button class="btn btn-preset" onclick="loadPreset('cascade')">🔗 级联</button>
+        <button class="btn btn-preset" onclick="loadPreset('adv')">🧪 高级</button>
       </div>
     </div>
   </div>
@@ -385,7 +389,81 @@ SELECT name, age FROM users
 WHERE id IN (
   SELECT user_id FROM orders
   WHERE amount = (SELECT MAX(amount) FROM orders)
-);`
+);`,
+  tx: `-- 事务：插入用户及其订单（全部成功才提交）
+-- 使用 db.transaction() API（这里有演示限制）
+
+-- 模拟事务场景：先查余额，再插入订单
+SELECT * FROM users WHERE id = '1';
+
+INSERT INTO orders VALUES ('o99', '1', 'Headphones', 199, 1);
+
+SELECT u.name, COUNT(*) as order_count, SUM(o.amount) as total
+FROM users u INNER JOIN orders o ON u.id = o.user_id
+WHERE u.id = '1'
+GROUP BY u.name;
+
+-- 回滚演示：记得用 db.transaction() 包裹哦`,
+  scalar: `-- 聚合无 GROUP BY → 单行结果
+SELECT COUNT(*) as total_users, AVG(age) as avg_age FROM users;
+
+-- 年龄大于平均值的用户（标量子查询）
+SELECT name, age FROM users
+WHERE age > (SELECT AVG(age) FROM users);
+
+-- 消费超过平均值的用户
+SELECT u.name, SUM(o.amount) as spent
+FROM users u INNER JOIN orders o ON u.id = o.user_id
+GROUP BY u.name
+HAVING SUM(o.amount) > (SELECT AVG(amount) FROM orders);`,
+  cascade: `-- 外键级联：先创建带外键的表
+DROP TABLE IF EXISTS orders;
+
+CREATE TABLE orders (
+  id STRING PRIMARY KEY,
+  user_id STRING REFERENCES users(id) ON DELETE CASCADE,
+  product STRING,
+  amount NUMBER,
+  quantity NUMBER DEFAULT 1
+);
+
+-- 重新插入测试数据
+INSERT INTO orders VALUES ('o1', '1', 'Laptop', 1200, 1);
+INSERT INTO orders VALUES ('o2', '1', 'Mouse', 50, 2);
+INSERT INTO orders VALUES ('o3', '2', 'Keyboard', 150, 1);
+
+-- 查看用户和订单
+SELECT u.name, o.product, o.amount
+FROM users u INNER JOIN orders o ON u.id = o.user_id;
+
+-- 删除用户1 → 其订单自动级联删除
+DELETE FROM users WHERE id = '1';
+
+-- 确认：只剩用户2的订单
+SELECT u.name, o.product, o.amount
+FROM users u LEFT JOIN orders o ON u.id = o.user_id;`,
+  adv: `-- 复杂 WHERE：年龄在20-30之间且名字不是Bob
+SELECT * FROM users
+WHERE age BETWEEN 20 AND 30 AND name != 'Bob'
+ORDER BY age DESC;
+
+-- DISTINCT 去重
+SELECT DISTINCT age FROM users ORDER BY age;
+
+-- 多表 JOIN + 过滤 + 排序 + 分页
+SELECT u.name, o.product, o.amount
+FROM users u
+INNER JOIN orders o ON u.id = o.user_id
+WHERE o.amount >= 50
+ORDER BY o.amount DESC
+LIMIT 5 OFFSET 0;
+
+-- 子查询 + 聚合混合
+SELECT u.name, (
+  SELECT COUNT(*) FROM orders o WHERE o.user_id = u.id
+) as order_count
+FROM users u
+WHERE u.age >= 25;`
 };
 
 function loadPreset(name) {