PostgreSQL with Deno - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the PostgreSQL client from Deno's standard library.

Deno

import { Client } from "https://deno.land/x/postgres@v[1]/mod.ts";

Drag options to blanks, or click blank then click option'

A0.17.0

B0.17.1

C0.18.0

D0.16.0

Attempts:

3 left

💡 Hint

Common Mistakes

Using an incorrect or outdated version number.

Forgetting to include the version in the import URL.

The correct version to import the PostgreSQL client is 0.17.1 for compatibility.

2fill in blank

medium

Complete the code to create a new PostgreSQL client instance with a connection string.

Deno

const client = new Client({ connectionString: "[1]" });

Drag options to blanks, or click blank then click option'

Asqlite://user:pass@localhost/dbname

Bmysql://user:pass@localhost:3306/dbname

Cpostgres://user:pass@localhost:5432/dbname

Dhttp://user:pass@localhost:5432/dbname

Attempts:

3 left

💡 Hint

Common Mistakes

Using a MySQL or SQLite connection string format.

Using HTTP protocol instead of PostgreSQL.

The connection string for PostgreSQL starts with 'postgres://'.

3fill in blank

hard

Fix the error in the async function to connect to the database.

Deno

async function connectDB() {
  await client.[1]();
}

Drag options to blanks, or click blank then click option'

Aconnect

Bopen

Cstart

Dinit

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'open' or 'start' which are not valid methods.

Forgetting to await the connection.

The correct method to connect the client is 'connect()'.

4fill in blank

hard

Fill both blanks to query all rows from the 'users' table and log the results.

Deno

const result = await client.[1]("SELECT * FROM users");
console.[2](result.rows);

Drag options to blanks, or click blank then click option'

AqueryArray

Bquery

Clog

Dprint

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'queryArray' which returns arrays but is less common here.

Using 'print' instead of 'log' for console output.

Use 'query' to run the SQL and 'log' to print the results.

5fill in blank

hard

Fill all three blanks to insert a new user into the 'users' table safely using parameters.

Deno

await client.[1]("INSERT INTO users (name, email) VALUES ($1, $2)", [2]);

Drag options to blanks, or click blank then click option'

Aquery

B["Alice", "[e-mail]"]

Cexecute

DqueryArray

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'execute' which is not a method in this client.

Passing parameters as a string instead of an array.

Use 'query' with parameter array to safely insert data.

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