docs: add some special keywords back

docs-cn/12-taos-sql/12-keywords/index.md

@@ -85,3 +85,44 @@ title: TDengine 参数限制与保留关键字
 | CONNECTIONS | HAVING     | NOT       | SOFFSET    | VNODES       |
 | CONNS       | ID         | NOTNULL   | STABLE     | WAL          |
 | COPY        | IF         | NOW       | STABLES    | WHERE        |
+| _C0         | _QSTART    | _QSTOP    | _QDURATION | _WSTART      |
+| _WSTOP      | _WDURATION |
+
+## 特殊说明
+### TBNAME
+`TBNAME` 可以视为超级表中一个特殊的标签，代表子表的表名。
+
+获取一个超级表所有的子表名及相关的标签信息：
+```mysql
+SELECT TBNAME, location FROM meters;
+
+统计超级表下辖子表数量：
+```mysql
+SELECT COUNT(TBNAME) FROM meters;
+```
+
+以上两个查询均只支持在WHERE条件子句中添加针对标签（TAGS）的过滤条件。例如：
+```mysql
+taos> SELECT TBNAME, location FROM meters;
+             tbname             |            location            |
+==================================================================
+ d1004                          | California.SanFrancisco        |
+ d1003                          | California.SanFrancisco        |
+ d1002                          | California.LosAngeles          |
+ d1001                          | California.LosAngeles          |
+Query OK, 4 row(s) in set (0.000881s)
+
+taos> SELECT COUNT(tbname) FROM meters WHERE groupId > 2;
+     count(tbname)     |
+========================
+                     2 |
+Query OK, 1 row(s) in set (0.001091s)
+```
+### _QSTART/_QSTOP/_QDURATION
+表示查询过滤窗口的起始，结束以及持续时间 (从2.6.0.0版本开始支持)
+
+### _WSTART/_WSTOP/_WDURATION
+窗口切分聚合查询（例如 interval/session window/state window）中表示每个切分窗口的起始，结束以及持续时间（从 2.6.0.0 版本开始支持）
+
+### _c0
+表示表或超级表的第一列
\ No newline at end of file

docs-en/12-taos-sql/12-keywords.md

@@ -46,3 +46,44 @@ There are about 200 keywords reserved by TDengine, they can't be used as the nam
 | CONNECTIONS | HAVING     | NOT       | SOFFSET    | VNODES       |
 | CONNS       | ID         | NOTNULL   | STable     | WAL          |
 | COPY        | IF         | NOW       | STableS    | WHERE        |
+| _C0         | _QSTART    | _QSTOP    | _QDURATION | _WSTART      |
+| _WSTOP      | _WDURATION |
+
+## Explanations
+### TBNAME
+`TBNAME` can be considered as a special tag, which represents the name of the subtable, in STable.
+
+Get the table name and tag values of all subtables in a STable.
+```mysql
+SELECT TBNAME, location FROM meters;
+
+Count the number of subtables in a STable.
+```mysql
+SELECT COUNT(TBNAME) FROM meters;
+```
+
+Only filter on TAGS can be used in WHERE clause in the above two query statements.
+```mysql
+taos> SELECT TBNAME, location FROM meters;
+             tbname             |            location            |
+==================================================================
+ d1004                          | California.SanFrancisco        |
+ d1003                          | California.SanFrancisco        |
+ d1002                          | California.LosAngeles          |
+ d1001                          | California.LosAngeles          |
+Query OK, 4 row(s) in set (0.000881s)
+
+taos> SELECT COUNT(tbname) FROM meters WHERE groupId > 2;
+     count(tbname)     |
+========================
+                     2 |
+Query OK, 1 row(s) in set (0.001091s)
+```
+### _QSTART/_QSTOP/_QDURATION
+The start, stop and duration of a query time window (Since version 2.6.0.0).
+
+### _WSTART/_WSTOP/_WDURATION
+The start, stop and duration of aggegate query by time window, like interval, session window, state window (Since version 2.6.0.0).
+
+### _c0
+The first column of a table or STable.
\ No newline at end of file