Reference

SQL Query Formatting Guide

Readable SQL is easier to debug, review, and change. A few simple habits go a long way.

SQL ignores whitespace, so formatting is purely for humans. Consistent layout makes it obvious what a query does and where to make a change.

Practice

Practice formatting a query

The habits that matter most

Put each major clause (SELECT, FROM, WHERE, GROUP BY, ORDER BY) on its own line.
One column per line in SELECT once you have more than two or three.
Indent JOIN and AND/OR conditions under their clause.
Use short, meaningful table aliases (c for customers, o for orders).
Pick one case convention. Lowercase keywords are common and easy to read.

Before

Hard to scan

select c.first_name,c.last_name,count(*) from customers c join orders o on o.customer_id=c.customer_id where c.is_active=true group by c.first_name,c.last_name having count(*)>2 order by count(*) desc

After

Same query, readable

select   c.first_name,
         c.last_name,
         count(*) as order_count
from     customers as c
join     orders    as o on o.customer_id = c.customer_id
where    c.is_active = true
group by c.first_name, c.last_name
having   count(*) > 2
order by order_count desc

Dialect note

The Playground has a Format button that tidies your query automatically (powered by sql-formatter in DuckDB mode). Handy for cleaning up a query you pasted in.

Lesson

Reading SQL Error Messages

Use error messages as clues.

Mistake

COUNT or SUM Too High After a JOIN

One-to-many joins can inflate totals.

Error

SQL Error Messages Explained

Paste an error message and find the fix, from GROUP BY to constraint failures.

Lesson

Debugging SQL Queries Step by Step

Shrink a query and check one piece at a time.