CTEs vs Subqueries in SQL: What’s the Difference and When to Use Them?

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Introduction

SQL allows you to write flexible and powerful queries using subqueries and Common Table Expressions (CTEs).
Both help break down complex logic, reuse code, and improve readability but they work a bit differently.

What is a Subquery?
A subquery is a query inside another query.
Below is a query which shows customers whose remaining amount is above the average.

SELECT 
  [Customer No_], 
  [Remaining Amount]
FROM [Customer Ledger Entry]
WHERE [Remaining Amount] > (
  SELECT AVG([Remaining Amount])
  FROM [Customer Ledger Entry]
);

What is a CTE (Common Table Expression)?
A CTE is a temporary result set you can reference in a main query.
It starts with the WITH keyword and improves readability, especially with multi-step logic.

-- Step 1: Define the CTE
WITH AvgBalance AS (
  SELECT 
    AVG([Remaining Amount]) AS avg_balance
  FROM [Customer Ledger Entry]
)

-- Step 2: Use the CTE in your main query
SELECT 
  [Customer No_],
  [Remaining Amount]
FROM [Customer Ledger Entry], AvgBalance
WHERE [Remaining Amount] > AvgBalance.avg_balance;

Conclusion:

Subqueries
Advantages Disadvantages
Quick and easy for simple filtering. Harder to read when nested.
Good for one-off checks. Redundancy if used multiple times (no reuse).

CTEs (Common Table Expressions)
Advantages Disadvantages
Clean, readable SQL for complex queries. May be slightly slower in some databases.
Can be recursive. Not supported in old SQL engines.

Both subqueries and CTEs help you write better SQL but choosing the right one depends on your needs.

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