Data Engineering Interview Questions: SQL and Databases II

In databases, efficient SQL queries are important for optimal performance. Poorly written queries can lead to slow response times and strain system resources. In this article, we’ll explore several best practices for writing efficient SQL queries, complete with examples to illustrate each point. Whether you’re an expert in database administration or just starting, these answers will help improve your life and be the best version of yourself, my apologies is improving your queries.

How do you optimise queries?

1. Joining Tables Efficiently

When joining multiple tables, the order and method of the join can significantly impact query performance.

Best Practices

• Place Smaller Tables First: Start your FROM clause with the smallest table.
• Largest Table as Base Table: The largest table should be considered the base table.
• Join Conditions: Place the base table on the right-hand side of the equal sign in the WHERE clause.

Example

Suppose we have three tables: Customers (small), Orders (medium), and OrderDetails (large).

SELECT od.OrderID, c.CustomerName, o.OrderDate
FROM…