name: sql-best-practices
description: SQL development best practices for writing efficient, secure, and maintainable database queries

SQL Best Practices

Core Principles

• Write clear, readable SQL with consistent formatting and meaningful aliases
• Prioritize query performance through proper indexing and optimization
• Implement security best practices to prevent SQL injection
• Use transactions appropriately for data integrity
• Document complex queries with inline comments

Query Writing Standards

Formatting and Style

• Use uppercase for SQL keywords (SELECT, FROM, WHERE, JOIN)
• Place each major clause on a new line for readability
• Use meaningful table aliases (e.g., customers AS c not customers AS x)
• Indent subqueries and nested conditions consistently
• Align column lists and conditions for visual clarity

SELECT
    c.customer_id,
    c.customer_name,
    o.order_date,
    o.total_amount
FROM customers AS c
INNER JOIN orders AS o ON c.customer_id = o.customer_id
WHERE o.order_date >= '2024-01-01'
    AND o.status = 'completed'
ORDER BY o.order_date DESC;

Column Selection

• Avoid SELECT * in production code; explicitly list required columns
• Use column aliases to clarify output: SELECT first_name AS "First Name"
• Consider the order of columns in SELECT for logical grouping

Filtering and Conditions

• Place most restrictive conditions first in WHERE clauses
• Use appropriate operators: prefer IN over multiple OR conditions
• Use EXISTS instead of IN for subqueries when checking existence
• Avoid functions on indexed columns in WHERE clauses when possible
• Use parameterized queries to prevent SQL injection

-- Preferred: Use EXISTS for existence checks
SELECT c.customer_name
FROM customers AS c
WHERE EXISTS (
    SELECT 1 FROM orders AS o
    WHERE o.customer_id = c.customer_id
    AND o.order_date > '2024-01-01'
);

-- Avoid: Function on indexed column
WHERE YEAR(order_date) = 2024

-- Preferred: Range comparison
WHERE order_date >= '2024-01-01' AND order_date < '2025-01-01'

Join Best Practices

• Always use explicit JOIN syntax instead of implicit joins in WHERE
• Specify join type explicitly (INNER, LEFT, RIGHT, FULL OUTER)
• Order joins from largest to smallest table when possible
• Use appropriate join types based on data requirements
• Be cautious with CROSS JOINs; ensure they are intentional

-- Explicit join (preferred)
SELECT c.name, o.order_id
FROM customers AS c
INNER JOIN orders AS o ON c.customer_id = o.customer_id;

-- Avoid implicit join
SELECT c.name, o.order_id
FROM customers c, orders o
WHERE c.customer_id = o.customer_id;

Performance Optimization

Indexing Guidelines

• Create indexes on columns used in WHERE, JOIN, and ORDER BY clauses
• Consider composite indexes for multi-column queries
• Avoid over-indexing; each index adds write overhead
• Regularly analyze and maintain indexes
• Use covering indexes for frequently executed queries

Query Optimization

• Use EXPLAIN/EXPLAIN ANALYZE to understand query execution plans
• Limit result sets with TOP/LIMIT when full results are not needed
• Use pagination for large result sets
• Avoid correlated subqueries when possible; use JOINs instead
• Consider query caching for frequently executed queries

-- Pagination example
SELECT product_id, product_name, price
FROM products
ORDER BY product_id
LIMIT 20 OFFSET 40;

Aggregation Best Practices

• Filter before grouping when possible (WHERE vs HAVING)
• Use appropriate aggregate functions (COUNT, SUM, AVG, etc.)
• Consider window functions for running totals and rankings

-- Efficient: Filter before aggregation
SELECT category_id, COUNT(*) AS product_count
FROM products
WHERE active = true
GROUP BY category_id
HAVING COUNT(*) > 10;

Transaction Management

• Keep transactions as short as possible
• Use appropriate isolation levels for your use case
• Always include error handling with ROLLBACK
• Avoid user interaction during open transactions
• Use savepoints for complex multi-step operations

BEGIN TRANSACTION;

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

IF @@ERROR <> 0
    ROLLBACK TRANSACTION;
ELSE
    COMMIT TRANSACTION;

Security Best Practices

• Always use parameterized queries or prepared statements
• Never concatenate user input directly into SQL strings
• Apply principle of least privilege for database users
• Audit and log sensitive data access
• Encrypt sensitive data at rest and in transit

-- Use parameterized queries (pseudo-code)
PREPARE stmt FROM 'SELECT * FROM users WHERE username = ?';
EXECUTE stmt USING @username;

Data Modification Best Practices

INSERT Operations

• Always specify column names explicitly
• Use bulk inserts for multiple rows when possible
• Consider using MERGE/UPSERT for insert-or-update scenarios

INSERT INTO customers (customer_name, email, created_at)
VALUES
    ('John Doe', '[email protected]', CURRENT_TIMESTAMP),
    ('Jane Smith', '[email protected]', CURRENT_TIMESTAMP);

UPDATE Operations

• Always include a WHERE clause (unless intentionally updating all rows)
• Test UPDATE queries with SELECT first
• Consider using transactions for critical updates

DELETE Operations

• Always include a WHERE clause
• Use soft deletes (status flags) for recoverable data
• Consider CASCADE effects on related tables

Naming Conventions

• Use snake_case for table and column names
• Use singular nouns for table names (customer, not customers)
• Prefix primary keys with table name: customer_id
• Use descriptive names: order_total not ot
• Prefix boolean columns appropriately: is_active, has_shipped

Documentation

• Comment complex business logic within queries
• Document stored procedures with purpose, parameters, and examples
• Maintain a data dictionary for table and column descriptions
• Version control database schema changes

Error Handling

• Implement proper error handling in stored procedures
• Log errors with sufficient context for debugging
• Return meaningful error messages to calling applications
• Use TRY-CATCH blocks where supported