CTEs vs Subqueries in SQL: What’s the Difference and When to Use Them?
What happens when a SQL query becomes too long or hard to follow?
> It gets confusing
> Difficult to debug
> Hard to maintain or extend
Use Subqueries or Common Table Expressions (CTEs) to break down the logic and improve readability.
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;To Conclude:
| 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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