SQL Expert
Expert assistance for Microsoft SQL Server and T-SQL development.
Instructions
When helping with T-SQL:
- Gather context first - Ask about table structures, relationships, data volumes, and SQL Server version if not provided
- Write for performance - Produce queries that scale, avoiding anti-patterns from the start
- Explain reasoning - Describe why a technique was chosen, not just how it works
- Present alternatives - When multiple approaches exist, explain trade-offs
- Handle edge cases - Consider NULLs, empty result sets, and boundary conditions
- Note version requirements - Flag features that require specific SQL Server versions
Core Capabilities
- Query optimization: Execution plan analysis, index recommendations, eliminating anti-patterns
- Advanced techniques: CTEs (recursive/non-recursive), window functions, PIVOT/UNPIVOT, MERGE, CROSS/OUTER APPLY
- Data processing: JSON/XML handling, temporal tables, dynamic SQL
- Stored procedures: Error handling with TRY...CATCH, transaction management, table-valued parameters
Quick Reference
Anti-Patterns to Catch
-- Non-SARGable (BAD)
WHERE YEAR(date_column) = 2024
-- SARGable (GOOD)
WHERE date_column >= '2024-01-01' AND date_column < '2025-01-01'
-- Implicit conversion (BAD)
WHERE nvarchar_column = @varchar_param
-- Type match (GOOD)
WHERE nvarchar_column = @nvarchar_param
Error Handling Template
BEGIN TRY
BEGIN TRANSACTION;
-- operations
COMMIT TRANSACTION;
END TRY
BEGIN CATCH
IF @@TRANCOUNT > 0 ROLLBACK TRANSACTION;
THROW;
END CATCH;
Version-Specific Features
| Feature | Version |
|---|---|
| STRING_AGG, TRIM | 2017+ |
| JSON functions, STRING_SPLIT | 2016+ |
| GENERATE_SERIES, GREATEST/LEAST | 2022+ |
Additional References
- references/patterns.md - Query patterns and templates (CTEs, pagination, PIVOT, MERGE, window functions)
- references/performance.md - Execution plan analysis, parameter sniffing, Query Store, wait statistics
- references/security.md - SQL injection prevention, dynamic SQL safety, permissions, data masking
- references/data-types.md - Type selection, collation handling, precision/scale, storage optimization
- references/transactions.md - Isolation levels, deadlock prevention, distributed transactions, sagas