Merge pull request #17956 from taosdata/docs/query_doc_wxy

docs: add examples of case expressions

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

@@ -334,6 +334,20 @@ If no comparison or condition is true, the result after ELSE is returned, or NUL
 
 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.
 
+### Examples
+
+A device has three status codes to display its status. The statements are as follows:
+
+```sql
+SELECT CASE dev_status WHEN 1 THEN 'Running' WHEN 2 THEN 'Warning' WHEN 3 THEN 'Downtime' ELSE 'Unknown' END FROM dev_table;
+```
+
+The average voltage value of the smart meter is counted. When the voltage is less than 200 or more than 250, it is considered that the statistics is wrong, and the value is corrected to 220. The statement is as follows:
+
+```sql
+SELECT AVG(CASE WHEN voltage < 200 or voltage > 250 THEN 220 ELSE voltage END) FROM meters;
+```
+
 ## 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/zh/12-taos-sql/06-select.md

@@ -325,14 +325,28 @@ CASE WHEN condition THEN result [WHEN condition THEN result ...] [ELSE result] E
 ```
 
 ### 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。
 
+### 示例
+
+某设备有三个状态码，显示其状态，语句如下：
 
+```sql
+SELECT CASE dev_status WHEN 1 THEN 'Running' WHEN 2 THEN 'Warning' WHEN 3 THEN 'Downtime' ELSE 'Unknown' END FROM dev_table;
+```
+
+统计智能电表的电压平均值，当电压小于 200 或大于 250 时认为是统计有误，修正其值为 220，语句如下：
+
+```sql
+SELECT AVG(CASE WHEN voltage < 200 or voltage > 250 THEN 220 ELSE voltage END) FROM meters;
+```
 
 ## JOIN 子句