Joins vs Split queries in SQL

Welcome to my newsletter! In “It Depends”, I’ll be discussing various trade-offs and challenges that software engineers face while working in teams and building real world applications.

Today’s article is about SQL joins - When should you use joins vs separate queries to fetch related data from a relational database?

Of course, it depends.

When you think relational databases, join queries may seem like the de-facto way to fetch data from multiple tables at once. In fact, the SQL database engines are optimized for it. However, with growing data volumes and complexity of your system, there are trade-offs to consider.

Sometimes, querying data separately and combining them in application code, although sounds like more work, can be an effective design choice.

Let’s explore the pros and cons between the two approaches:

Pros for joins:

✅ Fetching columns from multiple tables in a single query can provide lower network latency

✅ The DB engine can optimize join queries using indexed columns, yielding better performance

Cons for joins:

❌ While good for simple scenarios, complexity and volume of data can lead to higher execution times (higher latency) and more difficult maintainability

❌ Sub-optimal join queries on large datasets without proper indexing or relationship considerations can strain database resources, affecting scalability

Pros for separate queries:

✅ Code can be easier to maintain and debug since each query focuses on a specific task or requirement.

✅ Granular control over how and what data is fetched, allowing for more fine-tuned performance optimization

Cons for separate queries:

❌ Additional round trips between the application and the database server can potentially increase network latency / API slowness

❌ Can cause data inconsistency due to transactional isolations

To understand more about the value of each of these strategies, we must look at example use cases where they are applied. I will be publishing the full details of the examples on my Substack (https://siddiqus.substack.com)

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