docs: add case expressions

docs/en/12-taos-sql/06-select.md

@@ -314,6 +314,26 @@ Regular expression filtering is supported only on table names (TBNAME), BINARY t
 
 A regular expression string cannot exceed 128 bytes. You can configure this value by modifying the maxRegexStringLen parameter on the TDengine Client. The modified value takes effect when the client is restarted.
 
+## CASE Expressions
+
+### Syntax
+
+```txt
+CASE value WHEN compare_value THEN result [WHEN compare_value THEN result ...] [ELSE result] END
+CASE WHEN condition THEN result [WHEN condition THEN result ...] [ELSE result] END
+```
+
+### Description
+CASE expressions let you use IF ... THEN ... ELSE logic in SQL statements without having to invoke procedures. 
+
+The first CASE syntax returns the `result` for the first `value`=`compare_value` comparison that is true. 
+
+The second syntax returns the `result` for the first `condition` that is true. 
+
+If no comparison or condition is true, the result after ELSE is returned, or NULL if there is no ELSE part.
+
+The return type of the CASE expression is the result type of the first WHEN WHEN part, and the result type of the other WHEN WHEN parts and ELSE parts can be converted to them, otherwise TDengine will report an error.
+
 ## JOIN
 
 TDengine supports natural joins between supertables, between standard tables, and between subqueries. The difference between natural joins and inner joins is that natural joins require that the fields being joined in the supertables or standard tables must have the same name. Data or tag columns must be joined with the equivalent column in another table.

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

@@ -126,6 +126,12 @@ Only care about the information of the status window when the status is 2. For e
 SELECT * FROM (SELECT COUNT(*) AS cnt, FIRST(ts) AS fst, status FROM temp_tb_1 STATE_WINDOW(status)) t WHERE status = 2;
 ```
 
+TDengine also supports the use of CASE expressions in state quantities. It can express that the beginning of a state is triggered by meeting a certain condition, and the end of this state is triggered by meeting another condition. For example, if the normal voltage range of the smart meter is 205V to 235V, you can judge whether the circuit is normal by monitoring the voltage.
+
+```
+SELECT tbname, _wstart, CASE WHEN voltage >= 205 and voltage <= 235 THEN 1 ELSE 0 END status FROM meters PARTITION BY tbname STATE_WINDOW(CASE WHEN voltage >= 205 and voltage <= 235 THEN 1 ELSE 0 END);
+```
+
 ### Session Window
 
 The primary key, i.e. timestamp, is used to determine which session window a row belongs to. As shown in the figure below, if the limit of time interval for the session window is specified as 12 seconds, then the 6 rows in the figure constitutes 2 time windows, [2019-04-28 14:22:10，2019-04-28 14:22:30] and [2019-04-28 14:23:10，2019-04-28 14:23:30] because the time difference between 2019-04-28 14:22:30 and 2019-04-28 14:23:10 is 40 seconds, which exceeds the time interval limit of 12 seconds.

docs/zh/12-taos-sql/06-select.md

@@ -315,6 +315,25 @@ WHERE (column|tbname) **match/MATCH/nmatch/NMATCH** _regex_
 
 正则匹配字符串长度不能超过 128 字节。可以通过参数 _maxRegexStringLen_ 设置和调整最大允许的正则匹配字符串，该参数是客户端配置参数，需要重启才能生效。
 
+## CASE 表达式
+
+### 语法
+
+```txt
+CASE value WHEN compare_value THEN result [WHEN compare_value THEN result ...] [ELSE result] END
+CASE WHEN condition THEN result [WHEN condition THEN result ...] [ELSE result] END
+```
+
+### 说明
+TDengine 通过 CASE 表达式让用户可以在 SQL 语句中使用 IF ... THEN ... ELSE 逻辑。
+第一种 CASE 语法返回第一个 value 等于 compare_value 的 result，如果没有 compare_value 符合，则返回 ELSE 之后的 result，如果没有 ELSE 部分，则返回 NULL。
+
+第二种语法返回第一个 condition 为真的 result。 如果没有 condition 符合，则返回 ELSE 之后的 result，如果没有 ELSE 部分，则返回 NULL。
+
+CASE 表达式的返回类型为第一个 WHEN THEN 部分的 result 类型，其余 WHEN THEN 部分和 ELSE 部分，result 类型都需要可以向其转换，否则 TDengine 会报错。
+
+
+
 ## JOIN 子句
 
 TDengine 支持“普通表与普通表之间”、“超级表与超级表之间”、“子查询与子查询之间” 进行自然连接。自然连接与内连接的主要区别是，自然连接要求参与连接的字段在不同的表/超级表中必须是同名字段。也即，TDengine 在连接关系的表达中，要求必须使用同名数据列/标签列的相等关系。

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

@@ -119,6 +119,12 @@ SELECT COUNT(*), FIRST(ts), status FROM temp_tb_1 STATE_WINDOW(status);
 SELECT * FROM (SELECT COUNT(*) AS cnt, FIRST(ts) AS fst, status FROM temp_tb_1 STATE_WINDOW(status)) t WHERE status = 2;
 ```
 
+TDengine 还支持将 CASE 表达式用在状态量，可以表达某个状态的开始是由满足某个条件而触发，这个状态的结束是由另外一个条件满足而触发的语义。例如，智能电表的电压正常范围是 205V 到 235V，那么可以通过监控电压来判断电路是否正常。
+
+```
+SELECT tbname, _wstart, CASE WHEN voltage >= 205 and voltage <= 235 THEN 1 ELSE 0 END status FROM meters PARTITION BY tbname STATE_WINDOW(CASE WHEN voltage >= 205 and voltage <= 235 THEN 1 ELSE 0 END);
+```
+
 ### 会话窗口
 
 会话窗口根据记录的时间戳主键的值来确定是否属于同一个会话。如下图所示，如果设置时间戳的连续的间隔小于等于 12 秒，则以下 6 条记录构成 2 个会话窗口，分别是：[2019-04-28 14:22:10，2019-04-28 14:22:30]和[2019-04-28 14:23:10，2019-04-28 14:23:30]。因为 2019-04-28 14:22:30 与 2019-04-28 14:23:10 之间的时间间隔是 40 秒，超过了连续时间间隔（12 秒）。