feat: SQL 语法支持 BETWEEN AND、DROP TABLE IF EXISTS

src/query/ast.ts

@@ -103,6 +103,8 @@ export interface CreateTableStatement {
 export interface DropTableStatement {
   type: 'DROP_TABLE';
   name: string;
+  /** IF EXISTS — 表不存在时不报错 */
+  ifExists?: boolean;
 }
 
 // ---------------------------------------------------------------------------

src/query/executor.ts

@@ -234,6 +234,10 @@ export class QueryExecutor {
   }
 
   private async executeDropTable(stmt: DropTableStatement): Promise<void> {
+    if (stmt.ifExists) {
+      const exists = await this.engine.hasTable(stmt.name);
+      if (!exists) return; // IF EXISTS: 表不存在时静默返回
+    }
     return this.engine.dropTable(stmt.name);
   }