add index on litemall_coupon.code (#334)

litemall-db/sql/litemall_table.sql

@@ -234,7 +234,8 @@ CREATE TABLE `litemall_coupon` (
   `add_time` datetime DEFAULT NULL COMMENT '创建时间',
   `update_time` datetime DEFAULT NULL COMMENT '更新时间',
   `deleted` tinyint(1) DEFAULT '0' COMMENT '逻辑删除',
-  PRIMARY KEY (`id`)
+  PRIMARY KEY (`id`),
+  KEY `code` (`code`)
 ) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8mb4 COMMENT='优惠券信息及规则表';
 /*!40101 SET character_set_client = @saved_cs_client */;