Yes — MySQL fully allows JOINs between temporary tables and permanent tables. Temporary tables behave like normal tables within the same session, so you can join them using INNER JOIN, LEFT JOIN, RIGHT JOIN, etc. However, there are important limitations to be aware of.

How Temporary Tables Work in JOINs

1
Temporary tables exist only for the duration of your session.
2
They are stored either in memory or disk depending on size.
3
They can be joined with permanent tables exactly like normal tables.
4
Indexes can be created on temporary tables and are used by the optimizer.

Example: Joining a Temporary Table with a Permanent Table

Key Limitations

1
Temporary tables are visible only to the session that created them — no other session can join them.
2
You cannot create a foreign key that references or is referenced by a temporary table.
3
Temporary table names can shadow permanent tables with the same name (the temporary table takes precedence).
4
Replication with temporary tables may require careful handling in statement-based replication.

Performance Considerations

1
JOINs with temporary tables are generally fast, especially when the temporary table is small.
2
Adding indexes to temporary tables can significantly speed up JOINs.
3
Large temporary tables may spill to disk, increasing I/O cost.
4
MySQL optimizer treats them like normal tables, but cannot use statistics stored in the data dictionary — it relies on session-level statistics only.

Adding Indexes for Faster JOINs

Best Practices

1
Always index columns used in JOIN conditions.
2
Use temporary tables for staging, filtering, or transforming data before heavier JOIN operations.
3
Avoid extremely large temporary tables — consider a persistent staging table instead.
4
Choose names carefully to avoid conflicts with permanent tables.

Difficulty: 5/10

Topics: temporary tables, JOIN syntax, performance limits

Scenario Questions

0-2 years experience

1
Suppose you need to combine data from a temporary table you just created with a permanent orders table to generate a report. How would you write the JOIN in MySQL?
2
If you create a temporary table inside a stored procedure and then try to join it with a regular table outside that procedure, what will happen and why?

2-5 years experience

1
You added a temporary table to speed up a complex aggregation, but after deploying, the JOIN with the users table started timing out. Walk me through how you would diagnose and fix the issue.
2
During a code review, a teammate used a LEFT JOIN between a temporary table and a permanent table but forgot to add an index on the temporary table's join column. Explain the impact and how you would address it.

5-8 years experience

1
Our analytics pipeline creates temporary tables nightly and joins them with several large fact tables. What architectural considerations and MySQL limitations would you keep in mind to ensure the pipeline scales?
2
We need to run a multi-step ETL where each step creates a temporary table that is later joined with permanent tables. Discuss how MySQL's session scope and transaction behavior affect reliability and what design patterns you’d use.

8+ years experience

1
The company is migrating from MySQL to a distributed SQL system, but many existing reports rely on temporary‑table joins. How would you plan the migration to preserve functionality while minimizing performance regressions?
2
Cross‑team, we have services that generate temporary tables for ad‑hoc analysis and then join them with core business tables. What governance, naming, and lifecycle policies would you establish to avoid conflicts and resource leaks at scale?

Follow-up Questions

❓What happens if you try to reference a temporary table from a different session?
❓Can you add indexes to a temporary table after it has been created?
❓How does the MySQL optimizer treat temporary tables in join order decisions?