SQL Best Practices for Writing Maintainable and Efficient Queries

Use Aliases for Readability

Aliasing tables and columns improves readability, especially when working with multiple tables or complex expressions.

SELECT e.employee_id, e.first_name AS fname, d.department_name AS dname
FROM employees e
JOIN departments d ON e.department_id = d.department_id;

Aliasing avoids repetitive typing and makes it easier to distinguish between columns from different tables.

Avoid SELECT \*

Explicitly specifying columns instead of using SELECT * is a best practice for several reasons:

• Performance: Reduces data transfer overhead.
• Maintainability: Prevents breaking changes when table structures evolve.
• Clarity: Makes it clear what data is being used.

-- Good practice
SELECT employee_id, first_name, last_name
FROM employees;

-- Avoid
SELECT *
FROM employees;

Use Consistent and Meaningful Naming Conventions

Consistent naming conventions for tables, columns, and aliases make code easier to read and maintain.

Stick to one convention across the project and document it.

Leverage Indexes and Understand Query Plans

Indexes can drastically improve query performance. Always consider which columns are frequently used in WHERE, JOIN, and ORDER BY clauses.

-- Create an index on frequently queried column
CREATE INDEX idx_employee_lastname ON employees(last_name);

Use EXPLAIN or EXPLAIN ANALYZE to understand how your queries are executed and where performance can be improved.

Parameterize Queries to Prevent SQL Injection

Use parameterized queries or prepared statements instead of concatenating user input into SQL strings.

-- Using parameterized query in a typical ORM context
SELECT * FROM users WHERE username = :username AND password = :password;

This approach ensures security and can also improve performance through query plan reuse.

Normalize and Denormalize Strategically

Database normalization reduces data redundancy and ensures data integrity. However, denormalization can improve performance in read-heavy workloads.

Strive for a balance based on the specific use case and performance requirements.

Use Transactions for Data Integrity

Wrap multiple related operations in a transaction to ensure atomicity and consistency.

BEGIN TRANSACTION;

UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;

COMMIT;

If any operation fails, use ROLLBACK to revert changes and maintain data integrity.

Avoid Using Functions on Indexed Columns in WHERE Clauses

Applying functions to indexed columns in WHERE clauses can prevent the database from using indexes efficiently.

-- Inefficient: prevents index usage
SELECT * FROM employees WHERE UPPER(last_name) = 'SMITH';

-- Efficient: case-insensitive comparison
SELECT * FROM employees WHERE last_name ILIKE 'smith';

Be mindful of how your expressions affect index usage.

Use Common Table Expressions (CTEs) for Complex Logic

CTEs make complex queries more readable and modular.

WITH recent_sales AS (
    SELECT product_id, SUM(quantity) AS total_sold
    FROM sales
    WHERE sale_date >= CURRENT_DATE - INTERVAL '30 days'
    GROUP BY product_id
)
SELECT p.product_name, rs.total_sold
FROM recent_sales rs
JOIN products p ON rs.product_id = p.product_id;

CTEs are especially useful for recursive queries and breaking down subqueries.

Document and Comment Your Queries

Add comments to explain complex logic or business rules embedded in the query.

-- Get active customers with at least 100 points
SELECT customer_id, name
FROM customers
WHERE status = 'active' AND loyalty_points >= 100;

Documentation helps others (and future you) understand the intent behind the code.

Learn more with useful resources

• SQL Style Guide
• Use the Index, Luke!
• SQL Performance Explained