docs: merge SQL variables overview into DDL section (#2872)

Author: BohuTANG

docs/en/sql-reference/00-sql-reference/41-sql-variables.md

@@ -1,22 +1,57 @@
 ---
-title: Variable
+title: SQL Variables
+sidebar_label: SQL Variables
 ---
 
-This page provides a comprehensive overview of Variable operations in Databend, organized by functionality for easy reference.
+SQL variables allow you to store and manage temporary data within a session, making scripts more dynamic and reusable.
 
-## Variable Management
+## Variable Commands
 
 | Command | Description |
 |---------|-------------|
-| [SET](set-variable.md) | Creates or modifies a session or user variable |
-| [UNSET](unset-variable.md) | Removes a user-defined variable |
+| [SET VARIABLE](set-variable.md) | Creates or modifies a session or user variable. |
+| [UNSET VARIABLE](unset-variable.md) | Removes a user-defined variable. |
+| [SHOW VARIABLES](show-variables.md) | Displays current values of system and user variables. |
 
-## Variable Information
+The SHOW VARIABLES command also has a table function counterpart, [`SHOW_VARIABLES`](../../../20-sql-functions/17-table-functions/show-variables.md), which returns the same information in a tabular format for richer filtering and querying.
 
-| Command | Description |
-|---------|-------------|
-| [SHOW VARIABLES](show-variables.md) | Displays current values of system and user variables |
+## Querying with Variables
+
+You can reference variables in statements for dynamic value substitution or to build object names at runtime.
+
+### Accessing Variables with `$` and `getvariable()`
+
+Use the `$` symbol or the `getvariable()` function to embed variable values directly in a query.
+
+```sql title='Example:'
+-- Set a variable to use as a filter value
+SET VARIABLE threshold = 100;
+
+-- Use the variable in a query with $
+SELECT * FROM sales WHERE amount > $threshold;
+
+-- Alternatively, use the getvariable() function
+SELECT * FROM sales WHERE amount > getvariable('threshold');
+```
+
+### Accessing Objects with `IDENTIFIER`
+
+The `IDENTIFIER` keyword lets you reference database objects whose names are stored in variables, enabling flexible query construction. (Note: BendSQL does not yet support `IDENTIFIER`.)
+
+```sql title='Example:'
+-- Create a table with sales data
+CREATE TABLE sales_data (region TEXT, sales_amount INT, month TEXT) AS
+SELECT 'North', 5000, 'January' UNION ALL
+SELECT 'South', 3000, 'January';
+
+select * from sales_data;
+
+-- Set variables for the table name and column name
+SET VARIABLE table_name = 'sales_data';
+SET VARIABLE column_name = 'sales_amount';
 
-:::note
-Variables in Databend allow you to store and reuse values within a session or across sessions, making scripts more dynamic and reusable.
-:::
+-- Use IDENTIFIER to dynamically reference the table and column in the query
+SELECT region, IDENTIFIER($column_name)
+FROM IDENTIFIER($table_name)
+WHERE IDENTIFIER($column_name) > 4000;
+```

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

@@ -19,6 +19,10 @@ const siteRedirects = [
     from: '/guides/access-data-lake/delta',
     to: '/sql/sql-reference/table-engines/delta'
   },
+  {
+    from: '/sql/sql-reference/sql-variables',
+    to: '/sql/sql-commands/ddl/variable/'
+  },
   // AI Functions redirects - functions moved to external implementation
   {
     from: '/sql/sql-functions/ai-functions/ai-cosine-distance',