FEAT-007: Multi-Column USING Clause in JOINs

[ajitpratap0]

Summary

Extend USING clause to support multiple columns - currently limited to single column.

Problem

Current: JOIN table USING (col1)
Needed: JOIN table USING (col1, col2, col3)

This is SQL-92 standard but not implemented.

Action Items

1. Update parser to handle column list in USING clause
2. Modify AST to store list of columns instead of single column
3. Add tests for 1-10 columns in USING clause
4. Update documentation with examples
5. Ensure backward compatibility with single-column USING

Acceptance Criteria

• Multi-column USING works correctly
• Tests cover 1-10 columns
• SQL-92 compliance improved
• Backward compatible with existing single-column usage
• Documentation updated

Technical Details

Priority: Medium
Effort: Small (8h)
Phase: Phase 4 - Feature Development
Dependencies: None

Example SQL

-- Current (works)
SELECT * FROM users u
JOIN profiles p USING (user_id);

-- Needed (doesn't work yet)
SELECT * FROM orders o
JOIN shipments s USING (order_id, warehouse_id);