How do you write efficient and effective SQL queries?

Here are some tips to help you write great SQL queries: 

• Select only the necessary columns
  Instead of using SELECT *, specify the necessary columns in your query. This will not only reduce the amount of data being fetched but also make your queries more readable and maintainable.
• Limit the number of rows
  If you’re only interested in a certain number of rows, use the LIMIT keyword to reduce the amount of data returned. This can greatly improve query performance, especially when working with large tables.
• Avoid using SELECT DISTINCT
  SELECT DISTINCT can be resource-intensive, as it requires the database to process and remove duplicate rows. Use GROUP BY instead, or consider normalizing your data to eliminate duplicates.
• Use JOINs wisely
  When joining multiple tables, ensure you’re using the appropriate type of JOIN (INNER, LEFT, RIGHT, or FULL) based on your requirements. Additionally, try to minimize the number of tables being joined, as this can impact query performance.
• Filter data early
  Apply filters in your WHERE clause as early as possible to reduce the number of rows that need to be processed. This can significantly improve query performance.
• Opt for EXISTS over IN
  When checking for the existence of rows in another table, using EXISTS is generally more efficient than using IN. This is because EXISTS stops searching once a match is found, whereas IN continues to process all rows.
• Use aggregate functions
  Functions like COUNT, SUM, AVG, MIN, and MAX can help you retrieve summary information quickly and efficiently without having to process all rows in a table.
• Avoid using subqueries when possible
  Subqueries can sometimes be inefficient, especially if they are not well-optimized. Consider rewriting your query using JOINs or other methods if it results in better performance. 

By following these best practices, you’ll be well on your way to writing efficient and effective SQL queries that meet your data analysis needs.