SQL Best Practices: Avoid Common Mistakes and Write Efficient Queries

Although learning SQL syntax is essential, truly mastering the language requires understanding how to write effective and efficient queries.

Below is a reverse analysis of things not to do, guiding you to write clear, optimized, powerful SQL statements:

1. Avoid ambiguous column naming:

Don’t

CREATE TABLE table1 (id int , name varchar(50), value money);
SELECT id, name, value FROM table1;

Do

CREATE TABLE table1 (customer_id int , full_name varchar(50), purchase_amount money);
SELECT customer_id, full_name, purchase_amount FROM customer_orders;

Use descriptive and meaningful column names that clearly convey the data they represent, improving readability and reducing confusion, especially in complex queries or collaborative environments.

2. Do not select all columns (unless necessary):

Don’t SELECT * FROM table1; Do

SELECT customer_id, email, phone_number FROM customers;

Selecting all columns can be inefficient for large tables; specify only the columns you need to avoid unnecessary data transfer and improve query performance.

3. Avoid unnecessary joins:

Don’t

SELECT * FROM 
customer c 
JOIN 
order o
ON 
c.id = o.id
JOIN products p 
ON 
c.id = p.id

Do

SELECT 
  c.customer_id, 
  o.order_date, 
  p.product_name 
FROM customers c 
INNER JOIN orders o 
ON c.customer_id = o.customer_id 
INNER JOIN products p 
ON o.product_id = p.product_id;

Join tables only when you need related data; unnecessary joins can significantly slow query speed, especially on large datasets.

4. Beware of missing WHERE clause:

Don’t SELECT * FROM customers; Do

SELECT * FROM 
customers 
WHERE activ_ind = 'Y';

Without a WHERE clause you retrieve all rows, which can be inefficient; use WHERE to filter data based on specific conditions.

5. Do not forget ORDER BY:

Don’t SELECT * FROM products; Do

SELECT * FROM 
products 
ORDER BY product_name ASC;

Unless you explicitly specify order, results may appear in an unpredictable sequence; ORDER BY organizes results clearly.

6. Avoid ambiguous aliases:

Don’t

SELECT 
c.id AS customer, 
o.date AS order_date 
FROM customers c, orders o;

Do

SELECT 
c.customer_id AS customer_id, 
o.order_date 
FROM customers c 
INNER JOIN orders o 
ON c.customer_id = o.customer_id;

Use meaningful aliases for tables and columns, especially in joins or complex queries, to improve readability and avoid confusion.

7. Do not ignore error handling:

Don’t (No error handling) Do

BEGIN TRY  -- Your SQL statement here -- 
END TRY 
BEGIN CATCH  -- Handle potential errors here -- 
END CATCH

Implement error handling to manage unexpected situations such as invalid data or connection issues, ensuring queries do not terminate abruptly.

8. Bypass redundant subqueries:

Don’t

SELECT customer_id FROM customers WHERE id IN 
(SELECT customer_id FROM orders WHERE order_date > '2023-01-01');

Do

SELECT c.customer_id
FROM customers c
INNER JOIN orders o ON c.customer_id = o.customer_id
WHERE o.order_date > '2023-01-01';

Avoid nested subqueries when possible; they can be hard to read and less efficient. Use joins or direct filters instead.

9. Do not ignore comments and documentation:

Don’t (No comments or documentation) Do

-- This query retrieves all active customers with their recent orders

Include comments and documentation to clarify query intent and aid future maintenance.