It's likely that you have considered this if you have ever built a sophisticated SQL query in SQL Server:
“Should I use a CTE here or just dump it into a temp table?”
Great question, and you’re not alone. CTEs (Common Table Expressions) and Temporary Tables both let you to break complex logic down into steps, but they serve different purposes under the hood. In this post, we'll break it all down in plain English, with examples and use cases, so you can confidently select the best option for your SQL journey.
What Is a CTE?
A CTE (Common Table Expression) is like a temporary named result set. You define it once at the top of your query, and then treat it like a virtual table.
Types of CTEs
There are two types of temporary tables:
- Simple CTE: Used to give a name to a query result.
- Multiple CTE: You can define more than one in a single query.
- Recursive CTE: A CTE that refers to itself, helpful in working with hierarchical data.
Syntax & Example
WITH TopSales AS (
SELECT TOP 5 EmployeeId, SUM(SalesAmount) AS TotalSales
FROM Sales
GROUP BY EmployeeId
ORDER BY TotalSales DESC
)
SELECT * FROM TopSales;
Think of it like giving a nickname to a subquery. It’s elegant, readable, and vanishes as soon as the query finishes.
Behind the Scenes
- CTEs are not stored in memory.
- SQL Server optimizes them as part of the main query, just like inlining code.
- You can’t index a CTE or reference it multiple times in the same query unless you write a recursive CTE.
A Quick Peek into What More CTEs Can Do
So far, we’ve looked at simple CTEs — but they can do a bit more. Two cool things to know:
1. You can use many CTEs together
You can define many CTEs in a query by separating them with commas. It is useful when you want to create step-by-step logic in a clean, readable format.
Example:
WITH SalesSummary AS (
SELECT EmployeeId, SUM(SalesAmount) AS TotalSales
FROM Sales
GROUP BY EmployeeId
),
TopPerformers AS (
SELECT * FROM SalesSummary WHERE TotalSales > 50000
)
SELECT * FROM TopPerformers;
2. Recursive CTEs Exist Too!
CTEs can also reference themselves — this is called a recursive CTE. They're especially useful for hierarchical data like org charts, categories, or any "parent-child" structures.
Example:
WITH OrgChart AS (
SELECT EmployeeId, ManagerId FROM Employees WHERE ManagerId IS NULL
UNION ALL
SELECT e.EmployeeId, e.ManagerId
FROM Employees e
INNER JOIN OrgChart c ON e.ManagerId = c.EmployeeId
)
SELECT * FROM OrgChart;
Don't worry if it appears advanced; the idea is that this CTE will continue to repeat until the entire organizational hierarchy has been created. We'll go over this in further detail in future article!
What is a Temporary Table?
A temporary table is exactly what it sounds like: a table that exists just temporarily within your SQL Server session.
You use it when you wish to store and manipulate intermediate data without permanently storing it in your database.
Types of Temporary Tables
There are two types of temporary tables:
- Local Temporary Table (#Temp): Only visible within your session or procedure. It disappears automatically when your session ends.
- Global Temporary Table (##Temp): Visible to all sessions. These are less commonly used and should be handled with care, especially in multi-user environments.
Syntax & Example
Let’s say we want to find the top 5 employees with the highest total sales — here’s how you can do it using a temporary table:
CREATE TABLE #TopSales (
EmployeeId INT,
TotalSales MONEY
);
INSERT INTO #TopSales
SELECT TOP 5 EmployeeId, SUM(SalesAmount)
FROM Sales
GROUP BY EmployeeId
ORDER BY SUM(SalesAmount) DESC;
SELECT * FROM #TopSales;
This table behaves just like a regular table, you can:
- Insert and select data from it.
- Add indexes for faster performance.
- Join it with other tables.
- Even update or delete rows from it.
Scope and Lifetime
The scope and lifetime of temp tables are like:
Local temp tables (#Temp) vanish automatically when your session or procedure ends.
Global temp tables (##Temp) disappear only after all active sessions using them are closed.
Both are stored in the special tempdb system database behind the scenes.
Do They Impact Performance?
Temporary tables are quite useful, especially when working with huge data sets and needing to break down complex queries. However, like with everything else, overusing them or failing to clear up might result in performance concerns.
Tip: You can manually drop a temp table when you're done using it — though SQL Server will often do this for you.
DROP TABLE #TopSales;
Side Note: You might hear about "table variables" too, they’re similar to temp tables but have some key differences. We’ll save that comparison for another article.
CTE vs Temp Table: The Key Differences
| Feature | CTE | Temporary Table |
| Lifetime |
Only during the single query |
Lasts for the entire session or proc |
| Reusability |
Not reusable, you can only reference it once per query block (unless it's a recursive CTE). If you need to refer to it multiple times, use a temp table instead. |
Can be reused in multiple queries |
| Indexing |
Not possible |
Can add indexes |
| Performance |
Better for simple, readable queries |
Better for large or reused datasets |
| Debugging |
Harder, because CTE vanishes after a run |
Easier as you can SELECT at any point |
| Memory Usage |
Doesn’t physically store data |
Stores in tempdb |
Note on CTE Reusability
When to Use CTEs?
Use a CTE when:
- You want cleaner, more readable queries.
- You’re chaining multiple subqueries (especially recursive ones).
- The data is small or used once.
- You don’t need to index or update the result.
CTEs are like inline helpers, great for readability but not heavy lifting.
When to Use Temp Tables
Use a Temporary Table when:
- You need to reuse data across multiple steps.
- You need to index, sort, or update the intermediate result.
- The dataset is large and complex.
- You’re working inside stored procedures or batch jobs.
Performance Tips
CTEs are great for cleaner queries, but remember, If you use the same CTE multiple times in one query, SQL Server might recalculate it each time. That means extra work — and slower results.
Temporary Tables store data physically in the tempdb database. So if you're just doing something small, avoid using them — they take extra time and resources.
For tiny bits of temporary data, like a few rows or quick calculations, consider using a table variable instead:
DECLARE @MyTable TABLE (Id INT, Name NVARCHAR(50));
It’s lightweight and faster for simple, short-lived tasks.
Quick Recap: CTE vs Temp Table Cheat Sheet
- Use CTE for readability, short-lived logic, and recursion.
- Use Temp Table for reuse, indexing, and larger datasets.
Still unsure? Ask yourself:
“Do I need to reuse or manipulate the data later?”
If yes, go with a temp table.
Conclusion
Temporary tables and Common Table Expressions are both great tools; they just work better in different situations. Selecting the appropriate one can improve the readability, speed, and manageability of your SQL code in the future. Now it's your turn: Have you ever had to choose between creating a temporary table to store results and creating a brief, repeated query? How did you handle it? Or forward this to a colleague who is now immersed in crafting those enormous, intricate queries; they would appreciate it!