Have you ever struggled with complex SQL queries, particularly when dealing with union data and left joins? I know I have. Recently, I came across a Reddit post that got me thinking about this very topic. Let me share the story.
Imagine you have three tables: People, Boats, and Cars. Each table has an ‘id’ column, and the Boats and Cars tables have a ‘peopleId’ column that references the ‘id’ column in the People table. You want to write a query that returns a result set with each person’s details, along with their vehicles (both boats and cars). The twist? You need to limit the number of vehicles per person to 2, and order them by purchase date.
The Reddit user who posted this question was struggling to achieve this result in a single query. They could either get all vehicles for each person and then filter the results in their code, or run separate queries to limit the results to 2. But is there a way to do it all in one query?
The short answer is yes, but it requires some clever use of window functions and common table expressions (CTEs). I won’t go into the details of the solution here, but I encourage you to check out the original Reddit post for more information.
What I want to highlight is the importance of practicing complex SQL queries. It’s easy to get stuck in a routine of writing simple queries, but when you’re faced with a challenging problem, it’s essential to have the skills to tackle it. So, take some time to practice and learn from others. You never know when you’ll need to master a complex SQL query.
Do you have any favorite resources for learning complex SQL queries? Share them in the comments below!