Merge pull request #23605 from taosdata/doc/TS-4072

add doc for using default unit of interval

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

@@ -38,11 +38,16 @@ Aggregation by time window is supported in TDengine. For example, in the case wh
 window_clause: {
     SESSION(ts_col, tol_val)
   | STATE_WINDOW(col)
-  | INTERVAL(interval [, offset]) [SLIDING sliding] [FILL({NONE | VALUE | PREV | NULL | LINEAR | NEXT})]
+  | INTERVAL(interval_val [, offset]) [SLIDING (sliding_value)] [FILL({NONE | VALUE | PREV | NULL | LINEAR | NEXT})]
   | EVENT_WINDOW START WITH start_trigger_condition END WITH end_trigger_condition
 }
 ```
 
+Both interval_val and sliding_value are time durations which have 3 forms of representation.
+- INTERVAL(1s, 500a) SLIDING(1s), the unit char should be any one of a (millisecond), b (nanosecond), d (day), h (hour), m (minute), n (month), s (second), u (microsecond), w (week), y (year).
+- INTERVAL(1000, 500) SLIDING(1000), the unit will the same as the queried database, if there are more than one databases, higher precision will be used.
+- INTERVAL('1s', '500a') SLIDING('1s'), unit must be specified, no spaces allowed.
+
 The following restrictions apply:
 
 ### Other Rules

docs/zh/12-taos-sql/12-distinguished.md

@@ -44,7 +44,11 @@ window_clause: {
 }
 ```
 
-在上述语法中的具体限制如下
+其中，interval_val 和 sliding_val 都表示时间段, 语法上支持三种方式，举例说明如下:
+ - INTERVAL(1s, 500a) SLIDING(1s), 自带时间单位的形式，其中的时间单位是单字符表示, 分别为: a (毫秒), b (纳秒), d (天), h (小时), m (分钟), n (月), s (秒), u (微妙), w (周), y (年).
+ - INTERVAL(1000, 500) SLIDING(1000), 不带时间单位的形式，将使用查询库的时间精度作为默认时间单位，当存在多个库时默认采用精度更高的库.
+ - INTERVAL('1s', '500a') SLIDING('1s'), 自带时间单位的字符串形式，字符串内部不能有任何空格等其它字符.
+
 
 ### 窗口子句的规则