PostgreSQL with Deno

Introduction

PostgreSQL is a popular database to store data. Using it with Deno lets you build apps that save and get data easily.

You want to save user info like names and emails in a web app.

You need to fetch data from a database to show on a website.

You want to update or delete records in a database from your Deno app.

You are building a backend API that talks to a PostgreSQL database.

You want to run SQL queries safely inside your Deno project.

Syntax

Deno

import { Client } from "https://deno.land/x/postgres@v0.17.0/mod.ts";

const client = new Client({
  user: "user",
  password: "password",
  database: "dbname",
  hostname: "localhost",
  port: 5432
});

await client.connect();

const result = await client.queryObject("SELECT * FROM tablename");

console.log(result.rows);

await client.end();

Use Client from the postgres module to connect to the database.

Always connect() before queries and end() after finishing.

Examples

This example connects to a database called shopdb and fetches all rows from the products table.

Deno

import { Client } from "https://deno.land/x/postgres@v0.17.0/mod.ts";

const client = new Client({
  user: "alice",
  password: "secret",
  database: "shopdb",
  hostname: "localhost",
  port: 5432
});

await client.connect();

const result = await client.queryObject("SELECT * FROM products");

console.log(result.rows);

await client.end();

This example inserts a new user into the users table using parameters to keep data safe.

Deno

import { Client } from "https://deno.land/x/postgres@v0.17.0/mod.ts";

const client = new Client({
  user: "bob",
  password: "mypassword",
  database: "testdb",
  hostname: "db.example.com",
  port: 5432
});

await client.connect();

await client.queryObject("INSERT INTO users(name, email) VALUES($1, $2)", ["Bob", "[e-mail]"]);

await client.end();

Sample Program

This program connects to PostgreSQL, creates a table if missing, inserts a message, then fetches and prints all messages.

Deno

import { Client } from "https://deno.land/x/postgres@v0.17.0/mod.ts";

async function main() {
  const client = new Client({
    user: "deno_user",
    password: "deno_pass",
    database: "deno_db",
    hostname: "localhost",
    port: 5432
  });

  await client.connect();

  // Create table if not exists
  await client.queryObject(`
    CREATE TABLE IF NOT EXISTS messages (
      id SERIAL PRIMARY KEY,
      content TEXT NOT NULL
    )
  `);

  // Insert a message
  await client.queryObject("INSERT INTO messages(content) VALUES($1)", ["Hello from Deno!"]);

  // Fetch messages
  const result = await client.queryObject("SELECT id, content FROM messages");

  console.log("Messages in database:");
  for (const row of result.rows) {
    console.log(`ID: ${row.id}, Content: ${row.content}`);
  }

  await client.end();
}

main();

Important Notes

Make sure PostgreSQL server is running and credentials are correct.

Use parameterized queries (like $1, $2) to avoid SQL injection.

Always close the connection with client.end() to free resources.

Summary

PostgreSQL with Deno lets you store and get data in your apps.

Use the Client from the postgres module to connect and run queries.

Remember to connect before queries and close connection after.

Practice

1. What is the main purpose of using the Client from the postgres module in Deno?

easy

A. To manage file uploads in Deno

B. To create a web server in Deno

C. To connect to a PostgreSQL database and run queries

D. To handle user authentication in Deno

Solution

1. Step 1: Understand the role of Client in postgres module

   The Client is designed to establish a connection to a PostgreSQL database and execute SQL queries.

2. Step 2: Compare with other options

   Creating web servers, managing file uploads, and handling authentication are unrelated to the Client in the postgres module.

3. Final Answer:

   To connect to a PostgreSQL database and run queries -> Option C

4. Quick Check:

   Client connects to DB and runs queries = C [OK]

Hint: Remember: Client is for DB connection and queries [OK]

Common Mistakes:

• Confusing Client with web server modules
• Thinking Client handles file uploads
• Assuming Client manages user authentication

2. Which of the following is the correct way to import the PostgreSQL client in a Deno script?

easy

A. import * as Client from "postgres";

B. import Client from "postgres";

C. const Client = require('postgres');

D. import { Client } from "https://deno.land/x/postgres@v0.17.0/mod.ts";

Solution

1. Step 1: Identify correct import syntax for Deno modules

   Deno uses ES module syntax with full URLs for third-party modules, so import must include the URL and version.

2. Step 2: Check each option

   import { Client } from "https://deno.land/x/postgres@v0.17.0/mod.ts"; uses correct ES module import with URL and version. Options B, C, and D use Node.js style or incomplete imports not valid in Deno.

3. Final Answer:

   import { Client } from "https://deno.land/x/postgres@v0.17.0/mod.ts"; -> Option D

4. Quick Check:

   Deno imports use full URLs = A [OK]

Hint: Deno imports need full URL with version [OK]

Common Mistakes:

• Using Node.js require() syntax
• Omitting the full URL in import
• Using wildcard import incorrectly

3. Given the following Deno code snippet, what will be the output?

import { Client } from "https://deno.land/x/postgres@v0.17.0/mod.ts";

const client = new Client({ user: "user", database: "testdb" });
await client.connect();
const result = await client.queryObject("SELECT 1 AS number");
await client.end();
console.log(result.rows[0].number);

medium

A. undefined

B. 1

C. Error: Connection not established

D. null

Solution

1. Step 1: Analyze the query execution

   The code connects to the database, runs a query selecting the number 1 as 'number', then closes the connection.

2. Step 2: Understand the result object

   The queryObject method returns an object with a rows array. The first row's 'number' field contains 1, so logging it prints 1.

3. Final Answer:

   1 -> Option B

4. Quick Check:

   Query returns 1, printed output = 1 [OK]

Hint: Query returns 1, check rows[0].number for output [OK]

Common Mistakes:

• Forgetting to await client.connect()
• Accessing wrong property of result
• Not closing the client connection

4. Identify the error in this Deno PostgreSQL code snippet:

import { Client } from "https://deno.land/x/postgres@v0.17.0/mod.ts";

const client = new Client({ user: "user", database: "testdb" });
const result = await client.queryObject("SELECT NOW()");
await client.end();
console.log(result.rows);

medium

A. Missing await client.connect() before query

B. Incorrect import statement

C. Using queryObject instead of query

D. Calling client.end() before query

Solution

1. Step 1: Check connection setup

   The code creates a client but never calls await client.connect() before running the query.

2. Step 2: Understand consequences

   Without connecting, the query will fail because the client is not connected to the database.

3. Final Answer:

   Missing await client.connect() before query -> Option A

4. Quick Check:

   Must connect before queries = D [OK]

Hint: Always connect client before running queries [OK]

Common Mistakes:

• Skipping client.connect() call
• Calling client.end() too early
• Confusing queryObject with query

5. You want to insert multiple rows into a PostgreSQL table using Deno's postgres client efficiently. Which approach is best to avoid multiple separate queries and ensure all rows are inserted in one operation?

hard

A. Use a single parameterized query with multiple VALUES tuples

B. Call client.queryObject() multiple times, once per row

C. Run a loop with client.connect() and client.end() for each row

D. Insert rows one by one without parameters

Solution

1. Step 1: Understand batch insertion

   Inserting multiple rows in one query reduces overhead and improves performance.

2. Step 2: Evaluate options

   A uses a single parameterized query with multiple VALUES tuples, which is efficient and safe. B runs multiple queries, increasing overhead. C reconnects repeatedly, which is inefficient. D risks SQL injection and is inefficient.

3. Final Answer:

   Use a single parameterized query with multiple VALUES tuples -> Option A

4. Quick Check:

   Batch insert with one query = A [OK]

Hint: Batch inserts use one query with multiple VALUES [OK]

Common Mistakes:

• Running many separate queries instead of one
• Opening and closing connection repeatedly
• Not using parameterized queries risking SQL injection