FEAT: Support 5.1 - Handled empty order-by clause by defaulting it to SELECT NULL

[NandanaRaol]

This pull request refines the _as_sql_window method in mssql/compiler.py to improve clarity, ensure correctness, and enhance support for SQL window functions. Key changes include adding detailed comments, handling edge cases for ORDER BY, and improving the formatting of the final SQL statement.

Enhancements to _as_sql_window method:

• Enhanced handling of SQL clauses:
  • Added logic to handle cases where order_by compiles to an empty string by defaulting to ORDER BY (SELECT NULL).
  • Ensured proper compilation of PARTITION BY, ORDER BY, and frame clauses with detailed comments for better maintainability. [1] [2]

[Copilot]

Pull Request Overview

This PR updates the _as_sql_window method to handle empty or missing ORDER BY clauses by defaulting to ORDER BY (SELECT NULL), adds detailed inline comments for each SQL clause, and refines the formatting of the final window SQL statement.

• Default to ORDER BY (SELECT NULL) when order_by is missing or compiles to an empty string
• Added step-by-step comments for expression support, partitioning, ordering, and framing
• Streamlined template selection and SQL formatting

Comments suppressed due to low confidence (1)

mssql/compiler.py:156

• New fallback logic for empty or missing ORDER BY is introduced here; consider adding unit tests to verify behavior when order_by is None and when it compiles to an empty string.

# Handle ORDER BY clause if present.