PostgreSQL with Deno - Mini Project: Build & Apply

PostgreSQL with Deno

📖 Scenario: You are building a simple Deno application that connects to a PostgreSQL database to manage a list of books in a library.This project will guide you through setting up the database connection, configuring query parameters, fetching data, and displaying the results.

🎯 Goal: Create a Deno script that connects to a PostgreSQL database, queries a books table, and logs the titles of books published after a certain year.

📋 What You'll Learn

Use the deno-postgres module to connect to PostgreSQL

Create a connection configuration object with hostname, user, password, database, and port

Write a query to select book titles published after a given year

Log the resulting book titles to the console

💡 Why This Matters

🌍 Real World

Connecting to databases is essential for backend applications to store and retrieve data. Deno with PostgreSQL is a modern stack for building scalable services.

💼 Career

Understanding how to connect and query databases using Deno is valuable for backend developers working with modern JavaScript runtimes and relational databases.

1

Set up PostgreSQL connection configuration

Create a constant called config with the following properties: user set to "user123", password set to "pass123", database set to "library", hostname set to "localhost", and port set to 5432.

Deno

# Your code here

Hint

Use an object literal with the exact property names and values as specified.

2

Import Client and create a new client instance

Import Client from "https://deno.land/x/postgres@v0.17.0/mod.ts" and create a constant called client by passing the config object to new Client().

Deno

const config = {
  user: "user123",
  password: "pass123",
  database: "library",
  hostname: "localhost",
  port: 5432
};

// Your code here

Hint

Use the exact import URL and create the client with the config object.

3

Connect and query books published after 2010

Write an async function called fetchRecentBooks that connects the client, runs the query SELECT title FROM books WHERE year > 2010, and returns the rows from the query result.

Deno

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

const config = {
  user: "user123",
  password: "pass123",
  database: "library",
  hostname: "localhost",
  port: 5432
};

const client = new Client(config);

// Your code here

Hint

Remember to use await for asynchronous calls and return the rows from the query.

4

Call the function and log book titles

Call fetchRecentBooks() and then use for...of to loop over the returned rows. Inside the loop, log each book's title to the console. Finally, close the client connection with client.end().

Deno

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

const config = {
  user: "user123",
  password: "pass123",
  database: "library",
  hostname: "localhost",
  port: 5432
};

const client = new Client(config);

async function fetchRecentBooks() {
  await client.connect();
  const result = await client.queryObject("SELECT title FROM books WHERE year > 2010");
  return result.rows;
}

// Your code here

Hint

Use a promise then to handle the async function call and loop through the results.

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