SQL Server - Quick Reference
Deep Knowledge: Use mcp__documentation__fetch_docs with technology: aspnet-core for EF Core + SQL Server patterns.
Common T-SQL Patterns
-- Table creation
CREATE TABLE Users (
Id INT IDENTITY(1,1) PRIMARY KEY,
Name NVARCHAR(100) NOT NULL,
Email NVARCHAR(255) NOT NULL UNIQUE,
CreatedAt DATETIME2 DEFAULT GETUTCDATE(),
UpdatedAt DATETIME2 NULL,
IsActive BIT DEFAULT 1
);
-- Pagination (SQL Server 2012+)
SELECT * FROM Users
ORDER BY Name
OFFSET @Skip ROWS
FETCH NEXT @Take ROWS ONLY;
-- MERGE (upsert)
MERGE INTO Users AS target
USING (SELECT @Email AS Email, @Name AS Name) AS source
ON target.Email = source.Email
WHEN MATCHED THEN
UPDATE SET Name = source.Name, UpdatedAt = GETUTCDATE()
WHEN NOT MATCHED THEN
INSERT (Name, Email) VALUES (source.Name, source.Email);
-- CTE (Common Table Expression)
WITH ActiveUsers AS (
SELECT Id, Name, ROW_NUMBER() OVER (ORDER BY CreatedAt DESC) AS RowNum
FROM Users WHERE IsActive = 1
)
SELECT * FROM ActiveUsers WHERE RowNum BETWEEN 1 AND 10;
Indexes
-- Clustered (one per table, usually PK)
CREATE CLUSTERED INDEX IX_Users_Id ON Users(Id);
-- Non-clustered
CREATE NONCLUSTERED INDEX IX_Users_Email ON Users(Email);
-- Covering index (includes extra columns)
CREATE NONCLUSTERED INDEX IX_Users_Email_Include
ON Users(Email) INCLUDE (Name, CreatedAt);
-- Filtered index
CREATE NONCLUSTERED INDEX IX_Users_Active
ON Users(Email) WHERE IsActive = 1;
-- Check missing indexes
SELECT * FROM sys.dm_db_missing_index_details;
Stored Procedures
CREATE PROCEDURE sp_GetUsersByStatus
@IsActive BIT,
@PageSize INT = 10,
@PageNumber INT = 1
AS
BEGIN
SET NOCOUNT ON;
SELECT Id, Name, Email, CreatedAt
FROM Users
WHERE IsActive = @IsActive
ORDER BY Name
OFFSET (@PageNumber - 1) * @PageSize ROWS
FETCH NEXT @PageSize ROWS ONLY;
END;
Performance Tips
| Tip | Why |
|---|
Use NVARCHAR over VARCHAR for Unicode | Avoid encoding issues |
| Add indexes on WHERE/JOIN columns | Speed up queries |
Use SET NOCOUNT ON in procedures | Reduce network traffic |
Avoid SELECT * | Only fetch needed columns |
| Use parameterized queries | Prevent SQL injection, plan reuse |
Use DATETIME2 over DATETIME | Better precision, smaller storage |
Query Performance Analysis
-- Execution plan
SET STATISTICS IO ON;
SET STATISTICS TIME ON;
-- Run your query
SET STATISTICS IO OFF;
SET STATISTICS TIME OFF;
-- Expensive queries
SELECT TOP 10
qs.total_elapsed_time / qs.execution_count AS avg_elapsed,
qs.execution_count,
SUBSTRING(qt.text, 1, 200) AS query_text
FROM sys.dm_exec_query_stats qs
CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) qt
ORDER BY avg_elapsed DESC;
Anti-Patterns
| Anti-Pattern | Why It's Bad | Correct Approach |
|---|
SELECT * | Wastes I/O, breaks on schema change | List specific columns |
| Cursors for row-by-row | Very slow | Use set-based operations |
| No indexes on FKs | Slow JOINs | Add non-clustered indexes |
NOLOCK everywhere | Dirty reads | Use proper isolation levels |
| String concatenation in SQL | SQL injection | Use parameterized queries |
Quick Troubleshooting
| Issue | Likely Cause | Solution |
|---|
| Slow query | Missing index | Check execution plan |
| Deadlocks | Conflicting locks | Check sys.dm_tran_locks |
| Timeout | Long-running query | Optimize or increase timeout |
| Truncation error | Column too small | Check NVARCHAR length |
