1- Filter Early, Aggregate Late: Apply filtering conditions in the WHERE clause early in the query, and perform aggregations in the HAVING or SELECT clauses as needed.
2- Use table aliases with columns when you are joining multiple tables.
3- Never use SELECT, always mention the list of columns in the select clause before deploying the code.
4- Add useful comments wherever you write complex logic. Avoid too many comments.
5- Use joins instead of correlated subqueries when possible for better performance.
6- Create CTEs instead of multiple sub-queries, it will make your query easy to read.
7- Join tables using JOIN keywords instead of writing join condition in where clause for better readability.
8- Never use order by in sub queries, It will unnecessarily increase runtime. In fact, some databases don’t even allow you to do that.
9- If you know there are no duplicates in 2 tables, use UNION ALL instead of UNION for better performance.
