Complex data analysis often requires more than just one query. You might need to filter data, calculate some aggregates, then join that back to the original data, and then filter again.
You could write one massive, nested SQL query... or you could use Temporary Tables.
What is a Temporary Table?
A Temporary Table is a table that exists only for the duration of your database session. As soon as you close your connection (or log out), it disappears.
- Private: Only you can see your temp tables. Other users are not affected.
- Transient: They are automatically dropped when the session ends.
- Performance: They are often faster than Common Table Expressions (CTEs) for heavy workloads because they can be indexed.
Syntax
In most SQL dialects (PostgreSQL, SQLite, MySQL), the syntax is similar:
CREATE TEMP TABLE active_users AS
SELECT * FROM users WHERE last_login > '2025-01-01';
When to use Temp Tables vs. CTEs?
We love CTEs (WITH clauses), but Temp Tables have distinct advantages:
- Multiple Steps: If you need to perform 5-6 different transformations on the same data, a Temp Table is easier to debug.
- Indexing: You can add an index to a Temp Table! You generally cannot index a CTE.
- Reuse: If you need to reference the intermediate data multiple times in your analysis, a Temp Table doesn't need to be re-calculated each time.
Interactive Example
Let's do a multi-step analysis on customer orders.
- Find customers who ordered high-value items (Temp Table 1)
- Calculate their average spend (Temp Table 2)
- Compare them to the global average.
Cleaning Up
While temp tables drop automatically at the end of a session, it is good practice to explicitly drop them if you are running long scripts:
DROP TABLE IF EXISTS big_spenders;
Summary
Use Temporary Tables when your analysis is too complex for a single query, or when you need to improve performance by indexing intermediate results. They are a powerful tool for the SQL analyst's toolkit.