docs: Add ORDER BY expressions example file

Adds comprehensive examples demonstrating the new ORDER BY expression
support with aggregate rewriting.

Examples include:
- Basic ORDER BY with aggregates (SUM, COUNT, AVG, MIN, MAX)
- Multiple aggregates in ORDER BY
- Mix of simple columns and aggregates
- COUNT(*) with and without aliases
- Using --show-transformations to see rewriting in action

All 8 examples execute successfully and demonstrate how the
OrderByAliasTransformer automatically rewrites aggregate expressions
to use SELECT clause aliases.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: TimelordUK

examples/order_by_expressions.sql

@@ -0,0 +1,112 @@
+-- #! ../data/sales_data.csv
+
+-- ================================================================
+-- ORDER BY Expression Support Examples
+-- ================================================================
+-- Demonstrates the new ORDER BY expression support with automatic
+-- aggregate rewriting via the OrderByAliasTransformer.
+--
+-- The transformer automatically rewrites ORDER BY aggregate expressions
+-- to use aliases from the SELECT clause, enabling standard SQL syntax.
+-- ================================================================
+
+-- Example 1: Basic ORDER BY with aggregate (most common pattern)
+-- The SUM() in ORDER BY is automatically rewritten to use the 'total' alias
+SELECT
+    region,
+    SUM(sales_amount) AS total
+FROM sales
+GROUP BY region
+ORDER BY SUM(sales_amount) DESC;
+GO
+
+-- Example 2: COUNT(*) without explicit alias
+-- Transformer auto-generates an alias (expr_N) and uses it
+SELECT
+    region,
+    COUNT(*) AS count
+FROM sales
+GROUP BY region
+ORDER BY COUNT(*) DESC;
+GO
+
+-- Example 3: Multiple aggregates in ORDER BY
+-- Both aggregates are rewritten to use their respective aliases
+SELECT
+    region,
+    SUM(sales_amount) AS total_sales,
+    AVG(sales_amount) AS avg_sales
+FROM sales
+GROUP BY region
+ORDER BY SUM(sales_amount) DESC, AVG(sales_amount) ASC;
+GO
+
+-- Example 4: Mix of aggregate and simple column in ORDER BY
+SELECT
+    region,
+    product,
+    SUM(sales_amount) AS total
+FROM sales
+GROUP BY region, product
+ORDER BY region ASC, SUM(sales_amount) DESC;
+GO
+
+-- Example 5: Using MIN and MAX aggregates
+SELECT
+    region,
+    MIN(sales_amount) AS min_sale,
+    MAX(sales_amount) AS max_sale
+FROM sales
+GROUP BY region
+ORDER BY MAX(sales_amount) DESC, MIN(sales_amount) ASC;
+GO
+
+-- Example 6: COUNT without explicit alias in SELECT
+-- Parser auto-generates expr_N, transformer uses that
+SELECT
+    region,
+    COUNT(*)
+FROM sales
+GROUP BY region
+ORDER BY COUNT(*) DESC;
+GO
+
+-- Example 7: AVG with ORDER BY
+SELECT
+    product,
+    AVG(sales_amount) AS average_price
+FROM sales
+GROUP BY product
+ORDER BY AVG(sales_amount) DESC;
+GO
+
+-- ================================================================
+-- How It Works
+-- ================================================================
+--
+-- Before transformation:
+--   ORDER BY SUM(sales_amount) DESC
+--
+-- After transformation:
+--   ORDER BY total DESC
+--
+-- The OrderByAliasTransformer:
+-- 1. Finds all aggregates in SELECT clause
+-- 2. Maps them to their aliases (or generates aliases)
+-- 3. Rewrites matching aggregates in ORDER BY to use the aliases
+-- 4. Executor only sees simple column references
+--
+-- Use --show-transformations flag to see the rewriting in action!
+-- ================================================================
+
+-- Example 8: Testing with --show-transformations
+-- Run this with: ./target/release/sql-cli -f examples/order_by_expressions.sql --show-transformations
+SELECT
+    region,
+    COUNT(*) AS num_sales,
+    SUM(sales_amount) AS total,
+    AVG(sales_amount) AS average
+FROM sales
+GROUP BY region
+ORDER BY SUM(sales_amount) DESC, COUNT(*) DESC;
+GO