fix: ALTER TABLE DROP COLUMN preserves all row data — no longer drops+recreates table

src/query/executor.ts

@@ -300,16 +300,21 @@ export class QueryExecutor {
       if (schema.columns[stmt.column.name]) {
         throw new DatabaseError(`Column "${stmt.column.name}" already exists in table "${stmt.name}"`, 'COLUMN_EXISTS');
       }
+      // 直接在 schema 引用上添加列（已有行的该列值为 undefined/default）
       schema.columns[stmt.column.name] = astColumnToColumnDef(stmt.column);
     } else if (stmt.action === 'DROP') {
       if (!schema.columns[stmt.column.name]) {
         throw new DatabaseError(`Column "${stmt.column.name}" does not exist in table "${stmt.name}"`, 'COLUMN_NOT_FOUND');
       }
+      // 从 schema 引用上删除列定义（保留所有行数据）
       delete schema.columns[stmt.column.name];
+      // 清除已有行中该列的值（MemoryEngine 的 find 返回引用，delete 直接生效）
+      const rows = await this.engine.find(stmt.name, { table: stmt.name });
+      const colName = stmt.column.name;
+      for (const row of rows) {
+        if (colName in row) delete row[colName];
+      }
     }
-    // 重建表结构
-    await this.engine.dropTable(stmt.name);
-    await this.engine.createTable(schema);
   }
 
   private async executeTruncateTable(stmt: TruncateTableStatement): Promise<void> {