feat: SQL 语法支持 CREATE TABLE IF NOT EXISTS

- Parser 支持 CREATE TABLE IF NOT EXISTS 语法
- AST CreateTableStatement 新增 ifNotExists 字段
- Executor 表已存在时静默跳过
- 修复 demo.html Aria 预设报错

src/query/ast.ts

@@ -94,6 +94,8 @@ export interface CreateTableStatement {
   type: 'CREATE_TABLE';
   name: string;
   columns: ASTColumnDef[];
+  /** IF NOT EXISTS — 表已存在时不报错 */
+  ifNotExists?: boolean;
 }
 
 // ---------------------------------------------------------------------------

src/query/executor.ts

@@ -228,6 +228,11 @@ export class QueryExecutor {
   }
 
   private async executeCreateTable(stmt: CreateTableStatement): Promise<void> {
+    // IF NOT EXISTS: 表已存在时静默返回
+    if (stmt.ifNotExists) {
+      const exists = await this.engine.hasTable(stmt.name);
+      if (exists) return;
+    }
     const columns: Record<string, any> = {};
     for (const col of stmt.columns) columns[col.name] = astColumnToColumnDef(col);
     return this.engine.createTable(createSchema(stmt.name, columns));

src/sql/parser.ts

@@ -327,6 +327,16 @@ export class Parser {
   private parseCreateTable(): CreateTableStatement {
     this.expect(TokenType.CREATE);
     this.expect(TokenType.TABLE);
+
+    // IF NOT EXISTS（可选）
+    let ifNotExists = false;
+    if (this.curTokenIs(TokenType.IF)) {
+      this.nextToken();
+      this.expect(TokenType.NOT);
+      this.expect(TokenType.EXISTS);
+      ifNotExists = true;
+    }
+
     const tableName = this.expectIdentifier('table name');
     this.expect(TokenType.LPAREN);
 
@@ -338,7 +348,7 @@ export class Parser {
 
     this.expect(TokenType.RPAREN);
 
-    return { type: 'CREATE_TABLE', name: tableName, columns };
+    return { type: 'CREATE_TABLE', name: tableName, columns, ifNotExists: ifNotExists || undefined };
   }
 
   private parseColumnDef(): ASTColumnDef {