# First Query

> Run your first query and check the PostgreSQL server and psql client versions.

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LLMS index: [llms.txt](/en/llms.txt)

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The fastest way to confirm your PostgreSQL setup is working is to run a query that needs no tables. PostgreSQL is happy to evaluate expressions on their own — every `SELECT` does not need a `FROM`. We will use that property to verify three things in one go: the server is running, your client can reach it, and you have a recent enough version to follow along.

## What you'll learn {#learn}

- How to run a SQL file with `psql -f`
- A minimal `SELECT` with no table
- How to read the PostgreSQL version string
- The difference between client and server versions

## Example {#example}

```sql {num="1.1" caption="First Query"}
-- Simplest query: no tables, just a literal
SELECT 1;

-- Arithmetic and string expressions work without a FROM
SELECT 2 + 3 AS sum, 'hello, postgres' AS greeting;

-- Check your PostgreSQL server version
SELECT version();

-- A more readable version, when you only need the number
SHOW server_version;
```

`SELECT 1` returns a single row with one column — the integer `1`. The expression form (`SELECT 2 + 3, 'hello, postgres'`) shows that any expression can be the body of a `SELECT`; PostgreSQL evaluates and returns it. `version()` is a built-in function that returns the full version string (operating system, compiler, etc.), while `SHOW server_version` returns just the version number. Both report the connected server's version. `psql --version` reports the client version, which may differ.

## Run the example {#run}

To run this file from the repository root:

```text
$ psql -f source/first-query.sql postgres
 ?column?
----------
        1
(1 row)

 sum | greeting
-----+-----------------
   5 | hello, postgres
(1 row)

                                                    version
------------------------------------------------------------------------------------------------------------------------------
 PostgreSQL 17.x on x86_64-pc-linux-gnu, compiled by gcc ...
(1 row)

 server_version
----------------
 17.x
(1 row)
```

Your version string will vary. If you see output like above, everything is wired up.

## Common pitfalls {#pitfalls}

- *"could not connect to server"* — the daemon isn't running. Start it with `brew services start postgresql` (macOS), `sudo systemctl start postgresql` (Linux), or whatever your install uses.
- *"role 'you' does not exist"* — PostgreSQL doesn't know about your OS user as a database user. Create one with `createuser --superuser $(whoami)` from a shell that has access to the cluster.
- *"database 'postgres' does not exist"* — some installs (especially homebrew on macOS) don't create a default `postgres` database. Run `createdb postgres` once, or use `createdb pbe` and pass `pbe` as the database name to every command.

> [!TIP] Tip
> If you omit the database name, `psql` uses your OS username as the default. To use the `postgres` database explicitly: `psql -f source/first-query.sql postgres`. To pass a connection string instead, use `psql 'postgres://user:pass@host:5432/dbname'`.

> [!NOTE] Try it
> Run `psql -f source/first-query.sql` and confirm the output. Then connect interactively with `psql postgres` and type `SELECT now();` followed by Enter. Exit with `\q`.

## Downloads {#downloads}

SQL file: [first-query.sql](/examples/source/first-query.sql) · [All lesson files](/examples/examples.zip) · [Setup guide](/en/#prerequisites)