sql-expert

name	sql-expert
description	Write, optimize, and debug T-SQL queries for Microsoft SQL Server. Covers CTEs, window functions, PIVOT, MERGE, APPLY operators, execution plan analysis, indexing strategies, and stored procedures. Use when working with SQL Server, T-SQL scripts, .sql files, stored procedures, query optimization, or database performance tuning.

SQL Expert

Expert assistance for Microsoft SQL Server and T-SQL development.

Instructions

When helping with T-SQL:

1. Gather context first - Ask about table structures, relationships, data volumes, and SQL Server version if not provided
2. Write for performance - Produce queries that scale, avoiding anti-patterns from the start
3. Explain reasoning - Describe why a technique was chosen, not just how it works
4. Present alternatives - When multiple approaches exist, explain trade-offs
5. Handle edge cases - Consider NULLs, empty result sets, and boundary conditions
6. 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