docs: SQL-99 compliance gap analysis (FEAT-001)

Comprehensive analysis of SQL-99 standard compliance for issue #67.

Analysis Summary:
- Current compliance: ~80-85%
- Target compliance: 95%
- Gap: 15 missing features identified and prioritized
- Total effort: 222 hours across 3 phases
- Recommended approach: Phased implementation over 14-20 weeks

Key Findings:
- Strong foundation in core SQL-99 (SELECT, JOINs, CTEs, window functions)
- High-priority gaps: NULLS FIRST/LAST, FETCH/OFFSET, GROUPING SETS/ROLLUP/CUBE
- Medium-priority: FILTER clause, LATERAL joins, MERGE statement
- Low-priority: Transaction control, GRANT/REVOKE (execution layer)

Phase 1 (4-6 weeks, 50h): Quick wins
- NULLS FIRST/LAST, FETCH/OFFSET, COALESCE/NULLIF, TRUNCATE
- Target: 88-90% compliance

Phase 2 (6-8 weeks, 84h): Analytics features
- FILTER clause, GROUPING SETS, ROLLUP, CUBE, Frame EXCLUDE
- Target: 93-94% compliance

Phase 3 (4-6 weeks, 88h): Advanced features
- LATERAL joins, MERGE, basic Array support, TABLE constructor
- Target: 95-96% compliance

Document includes:
- Detailed feature-by-feature analysis
- Implementation recommendations with code examples
- Effort estimates and risk assessment
- Testing strategies and quality gates
- SQL-99 standard references

No code implementation - research and documentation only as requested.

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

Author: 2 people