Can you JOIN a CSV file to a PostgreSQL table in SQL?

Not with a standard PostgreSQL client, because the file isn't a table in the database. The usual options are COPY/\copy the CSV into a staging table first, or use a foreign data wrapper like file_fdw. Varan avoids both: it exposes the CSV as a queryable table and joins it to your PostgreSQL table in one query, run locally on a DuckDB engine.

Do I have to import the CSV into PostgreSQL first?

No. With Varan the CSV stays a file on disk. The join reads from the file and from PostgreSQL at query time, so there is nothing to import and nothing written back to the database.

Does this work for Excel files too?

Yes. Varan treats CSV and Excel files as queryable tables, so you can join an .xlsx sheet to a PostgreSQL table the same way.

Varan Join the beta →

Varan › How to join a CSV to PostgreSQL

How to join a CSV file to a PostgreSQL table in one query

You have a table in PostgreSQL and a CSV on your desktop, and you need them together. Here's how to JOIN them in a single SQL query — without importing the file or standing up a pipeline.

Varan joining a PostgreSQL orders table to two CSV files in one SQL query, returning revenue by customer tier — A PostgreSQL `orders` table joined to two CSV files in one query — run locally on DuckDB.

The usual (painful) options

\copy into a staging table — load the CSV into PostgreSQL first, then join. Works, but you own a throwaway table and the data is a stale copy.
file_fdw foreign data wrapper — powerful, but needs server-side setup, superuser rights, and a fixed file path on the database host.
Export & merge by hand — dump the table to CSV and stitch in Excel or pandas. Type clashes and not repeatable.

What you actually want is to treat the file as a table and write a normal join.

The clean way: a local cross-source engine

Varan runs a local-first DuckDB engine that can read your PostgreSQL database and your CSV files at the same time, so a single JOIN spans both — read-only, on your machine, with nothing written back.

Step by step

Connect PostgreSQL. Add your database with read-only credentials.
Open the CSV. Drag the file in — Varan exposes it as a queryable table (and scans it for anomalies on open).
Write one query. Join the PostgreSQL table to the CSV table like any normal SQL:

SELECT c.name, c.country, s.tier,
       COUNT(o.id)  AS orders,
       SUM(o.total) AS revenue
FROM   customers  c                       -- customers.csv on disk
JOIN   pg__orders o ON o.customer_id = c.id  -- PostgreSQL table
JOIN   segments   s ON s.customer_id = c.id  -- segments.csv on disk
GROUP BY c.name, c.country, s.tier
ORDER BY revenue DESC;

Run it. The engine pulls only what it needs from PostgreSQL and reads the CSV directly — one joined result, in milliseconds.

Do this with your own file

Varan is a desktop app for macOS, Windows and Linux — free during the beta.

Request beta access →

FAQ

Can you JOIN a CSV to a PostgreSQL table without importing it?

Yes — Varan reads the file at query time and joins it to PostgreSQL live, so there's no import and no staging table.

What about joining a CSV to MySQL, or two databases together?

Same approach — see how to join tables from different databases.

How is this different from a normal SQL client?

A normal client connects to one database and can't see a file as a table. See Varan vs DBeaver and Varan vs TablePlus.