enhance SET VARIABLE documentation with detailed examples

Author: BohuTANG

docs/en/sql-reference/10-sql-commands/00-ddl/15-variable/set-variable.md

@@ -5,7 +5,7 @@ import FunctionDescription from '@site/src/components/FunctionDescription';
 
 <FunctionDescription description="Introduced or updated: v1.2.609"/>
 
-Sets the value of one or more SQL variables within a session. The values can be simple constants, expressions, query results, or database objects.
+Sets the value of one or more SQL variables within a session. The values can be simple constants, expressions, query results, or database objects. Variables persist for the duration of your session and can be used in subsequent queries.
 
 ## Syntax
 
@@ -15,43 +15,78 @@ SET VARIABLE <variable_name> = <expression>
 
 -- Set more than one variable
 SET VARIABLE (<variable1>, <variable2>, ...) = (<expression1>, <expression2>, ...)
+
+-- Set multiple variables from a query result
+SET VARIABLE (<variable1>, <variable2>, ...) = <query>
 ```
 
+## Accessing Variables
+
+Variables can be accessed using the dollar sign syntax: `$variable_name`
+
 ## Examples
 
-The following example sets a single variable:
+### Setting a Single Variable
 
 ```sql
--- sets variable a to the string 'databend'
+-- Sets variable a to the string 'databend'
 SET VARIABLE a = 'databend'; 
+
+-- Access the variable
+SELECT $a;
+┌─────────┐
+│ $a      │
+├─────────┤
+│ databend│
+└─────────┘
 ```
 
-The following example sets a variable with the table name and uses IDENTIFIER to dynamically query the table based on that variable:
+### Setting Multiple Variables
 
 ```sql
-CREATE TABLE monthly_sales(empid INT, amount INT, month TEXT) AS SELECT 1, 2, '3';
-
--- Set a variable 't' to the name of the table 'monthly_sales'
-SET VARIABLE t = 'monthly_sales';
-
--- Use IDENTIFIER to dynamically reference the table name stored in the variable 't'
-SELECT * FROM IDENTIFIER($t);
+-- Sets variable x to 'xx' and y to 'yy'
+SET VARIABLE (x, y) = ('xx', 'yy');
 
-empid|amount|month|
------+------+-----+
-    1|     2|3    |
+-- Access multiple variables
+SELECT $x, $y;
+┌────┬────┐
+│ $x │ $y │
+├────┼────┤
+│ xx │ yy │
+└────┴────┘
 ```
 
-The following example sets multiple variables from a query in a single statement. The query must return exactly one row, with the same number of values as the variables being set.
+### Setting Variables from Query Results
 
 ```sql
 -- Sets variable a to 3 and b to 55
 SET VARIABLE (a, b) = (SELECT 3, 55); 
+
+-- Access the variables
+SELECT $a, $b;
+┌────┬────┐
+│ $a │ $b │
+├────┼────┤
+│ 3  │ 55 │
+└────┴────┘
 ```
 
-The following example sets multiple variables to constants:
+### Dynamic Table References
+
+Variables can be used with the `IDENTIFIER()` function to dynamically reference database objects:
 
 ```sql
--- Sets variable x to 'xx' and y to 'yy'
-SET VARIABLE (x, y) = ('xx', 'yy');
-```
+-- Create a sample table
+CREATE OR REPLACE TABLE monthly_sales(empid INT, amount INT, month TEXT) AS SELECT 1, 2, '3';
+
+-- Set a variable 't' to the name of the table 'monthly_sales'
+SET VARIABLE t = 'monthly_sales';
+
+-- Use IDENTIFIER to dynamically reference the table name stored in the variable 't'
+SELECT * FROM IDENTIFIER($t);
+┌───────┬────────┬───────┐
+│ empid │ amount │ month │
+├───────┼────────┼───────┤
+│     1 │      2 │ 3     │
+└───────┴────────┴───────┘
+```