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 with live documentation verification.

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+

Workflow

1. Query Development

Follow T-SQL best practices:

• Parameterized queries with sp_executesql for dynamic SQL
• Appropriate data types matching column definitions
• SARGable predicates for index utilization
• TRY...CATCH with proper transaction handling

See references/patterns.md for query templates.

2. Performance Optimization

Analyze and optimize query performance:

• Execution plan analysis for operator costs
• Index recommendations from missing index DMVs
• Parameter sniffing detection and solutions
• Query Store for regression analysis

See references/performance.md for tuning techniques.

3. Security Implementation

Protect against SQL injection and enforce least privilege:

• Always use sp_executesql with parameters
• QUOTENAME for dynamic object names
• Row-level security for multi-tenant
• Dynamic data masking for sensitive columns

See references/security.md for security patterns.

Live Verification

Do not rely solely on training data for exact T-SQL syntax, parameter lists, or version-specific behavior. Use WebFetch and WebSearch to verify against official Microsoft documentation.

When to Verify

MUST verify — exact function signatures, parameter names/types, return types, version-introduced annotations SHOULD verify — version-specific feature availability when user specifies a SQL Server version different from 2019+ Skip verification — general best practices, fundamental SQL syntax (SELECT, JOIN, WHERE), patterns covered in bundled references

Documentation Sources (Raw Markdown)

Use WebFetch with these URL patterns to retrieve raw documentation:

Content Type	URL Pattern
Functions	https://raw.githubusercontent.com/MicrosoftDocs/sql-docs/live/docs/t-sql/functions/{function-name}-transact-sql.md
Statements	https://raw.githubusercontent.com/MicrosoftDocs/sql-docs/live/docs/t-sql/statements/{statement-name}-transact-sql.md
Data Types	https://raw.githubusercontent.com/MicrosoftDocs/sql-docs/live/docs/t-sql/data-types/{type-name}-transact-sql.md
Language Elements	https://raw.githubusercontent.com/MicrosoftDocs/sql-docs/live/docs/t-sql/language-elements/{element-name}-transact-sql.md

Example: To verify STRING_AGG, use WebFetch on: https://raw.githubusercontent.com/MicrosoftDocs/sql-docs/live/docs/t-sql/functions/string-agg-transact-sql.md

Verification Workflow

1. Try WebFetch on the raw GitHub URL using the patterns above. Confirm the function signature, parameters, return type, and version requirements from the fetched content.
2. Fallback to WebSearch if the URL returns an error or the content is unclear. Search: {function-name} T-SQL site:learn.microsoft.com/en-us/sql. Then use WebFetch on the result URL.
3. State uncertainty if neither tool provides a clear answer:

   "I wasn't able to verify this syntax against live documentation. Please confirm at: https://learn.microsoft.com/en-us/sql/t-sql/functions/{function-name}"

What to Extract from Verified Docs

After fetching documentation, confirm and include:

• Complete syntax with all optional clauses
• Required vs optional parameters
• Return type
• SQL Server version where the feature was introduced
• Any deprecation warnings or behavior changes across versions

Note any discrepancies between training knowledge and live docs — the live documentation is authoritative.

Key Patterns

Pagination

-- Offset-fetch (SQL Server 2012+)
SELECT columns FROM table
ORDER BY sort_column
OFFSET @PageSize * (@PageNumber - 1) ROWS
FETCH NEXT @PageSize ROWS ONLY;

Running Totals

SELECT column, amount,
    SUM(amount) OVER (ORDER BY date_column ROWS UNBOUNDED PRECEDING) AS running_total
FROM table;

Safe Dynamic SQL

DECLARE @sql NVARCHAR(MAX) = N'SELECT * FROM Users WHERE Name = @Name';
EXEC sp_executesql @sql, N'@Name NVARCHAR(100)', @Name = @UserInput;

References

• references/patterns.md - CTEs, pagination, PIVOT, MERGE, window functions, APPLY operators
• 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

Documentation Resources

• SQL Server Docs: https://learn.microsoft.com/en-us/sql/
• T-SQL Reference: https://learn.microsoft.com/en-us/sql/t-sql/language-reference
• GitHub Docs (Raw Markdown): https://github.com/MicrosoftDocs/sql-docs