SQL Query Optimization Best Practices

Optimizing SQL queries is crucial for application performance. Here are essential techniques.

1. Use Proper Indexing

Indexes are the foundation of query performance:

-- Create index on frequently queried columns
CREATE INDEX idx_user_email ON Users(Email);

-- Composite index for multiple columns
CREATE INDEX idx_order_customer_date ON Orders(CustomerId, OrderDate);

2. Avoid SELECT *

Always specify the columns you need:

-- Bad
SELECT * FROM Users WHERE Id = 1;

-- Good
SELECT Id, Name, Email FROM Users WHERE Id = 1;

3. Use WHERE Instead of HAVING

Filter data as early as possible:

-- Less efficient
SELECT CustomerId, COUNT(*) as OrderCount
FROM Orders
GROUP BY CustomerId
HAVING CustomerId > 100;

-- More efficient
SELECT CustomerId, COUNT(*) as OrderCount
FROM Orders
WHERE CustomerId > 100
GROUP BY CustomerId;

4. Optimize JOINs

Use INNER JOIN when possible
Ensure JOIN columns are indexed
Consider denormalization for frequently joined tables

5. Use Query Execution Plans

Analyze your queries using execution plans to identify bottlenecks:

-- SQL Server
SET SHOWPLAN_ALL ON;
GO
-- Your query here
GO
SET SHOWPLAN_ALL OFF;

6. Avoid Functions on Indexed Columns

Functions prevent index usage:

-- Bad (index not used)
SELECT * FROM Users WHERE YEAR(CreatedDate) = 2024;

-- Good (index used)
SELECT * FROM Users 
WHERE CreatedDate >= '2024-01-01' 
AND CreatedDate < '2025-01-01';

Regular monitoring and optimization of your SQL queries will ensure your application remains performant as data grows.