When to Use CTEs and When to Use Subqueries in SQL?

Knowing the right tools to use for different scenarios is crucial for writing efficient and readable queries. Here’s a quick guide on when to use Common Table Expressions (CTEs) versus subqueries:

𝐂𝐨𝐦𝐦𝐨𝐧 𝐓𝐚𝐛𝐥𝐞 𝐄𝐱𝐩𝐫𝐞𝐬𝐬𝐢𝐨𝐧𝐬 (𝐂𝐓𝐄𝐬):

🔹 𝐃𝐞𝐟𝐢𝐧𝐢𝐭𝐢𝐨𝐧: Temporary result set defined at the beginning of your query, often making your code modular and easier to debug.

🔹 𝐑𝐞𝐮𝐬𝐞: Can be referenced multiple times within the main query.

🔹 𝐑𝐞𝐚𝐝𝐚𝐛𝐢𝐥𝐢𝐭𝐲: Greatly enhances code readability.

🔹 𝐔𝐬𝐚𝐠𝐞: Ideal for breaking up complicated queries into manageable parts.

𝐒𝐮𝐛𝐪𝐮𝐞𝐫𝐢𝐞𝐬:

🔹 𝐃𝐞𝐟𝐢𝐧𝐢𝐭𝐢𝐨𝐧: Nested mini-queries that execute within the main query.

🔹 𝐈𝐧𝐥𝐢𝐧𝐞 𝐖𝐫𝐢𝐭𝐢𝐧𝐠: Can be used directly within various clauses like WHERE, SELECT, or FROM.

🔹 𝐐𝐮𝐢𝐜𝐤 𝐅𝐢𝐥𝐭𝐞𝐫𝐢𝐧𝐠: Perfect for filtering or checks, such as verifying if a value exists in another table, list of values (table subqueries), or even entire result sets.

🔹 𝐂𝐨𝐦𝐩𝐥𝐞𝐱𝐢𝐭𝐲: If overused, can make code less readable. There are many types in Sub-Queries, a post coming soon on that:)

𝐄𝐱𝐚𝐦𝐩𝐥𝐞 𝐔𝐬𝐞 𝐂𝐚𝐬𝐞𝐬:

1. 𝐂𝐓𝐄: When performing a series of complex transformations and joins, defining intermediate steps as CTEs can make your main query much clearer and easier to maintain.

2. 𝐒𝐮𝐛𝐪𝐮𝐞𝐫𝐲: When you need to filter results based on the existence of a related record in another table. Choosing between a 𝐂𝐓𝐄 and a 𝐬𝐮𝐛𝐪𝐮𝐞𝐫𝐲 often comes down to readability and reusability. Master both to write cleaner, more efficient SQL code!

I have attached an example code below for you to check out!

Also, which one do you like or use the most? I like CTEs, haha.