SQL Server - Quick Reference
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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 |
Source: claude-dev-suite/claude-dev-suite — distributed by TomeVault.