Executing queries with query method in PHP - Mini Project: Build & Apply

Executing queries with query method

📖 Scenario: You are building a simple PHP script to fetch user data from a database. You will practice running SQL queries using the query method.

🎯 Goal: Create a PHP script that connects to a database, runs a SELECT query using the query method, and displays the user names.

📋 What You'll Learn

Create a mysqli connection object named $conn with host 'localhost', user 'root', password '', and database 'testdb'.

Create a SQL query string $sql to select all columns from the users table.

Use the query method on $conn with $sql to get the result in $result.

Loop through $result using fetch_assoc() and print each user's name.

Close the database connection at the end.

💡 Why This Matters

🌍 Real World

Fetching data from a database is a common task in web development to show dynamic content like user profiles or product lists.

💼 Career

Understanding how to execute queries and handle results is essential for backend developers working with PHP and MySQL databases.

1

Create the database connection

Create a mysqli connection object called $conn using new mysqli("localhost", "root", "", "testdb").

PHP

<?php
// Create the mysqli connection object $conn
// Your code here

Hint

Use the new mysqli(host, user, password, database) syntax to connect.

2

Write the SQL query string

Create a string variable called $sql and set it to "SELECT * FROM users".

PHP

<?php
$conn = new mysqli("localhost", "root", "", "testdb");
// Create the SQL query string $sql
// Your code here

Hint

Assign the exact string "SELECT * FROM users" to $sql.

3

Run the query using the query method

Use the query method on $conn with $sql and store the result in $result.

PHP

<?php
$conn = new mysqli("localhost", "root", "", "testdb");
$sql = "SELECT * FROM users";
// Run the query and store the result in $result
// Your code here

Hint

Use $conn->query($sql) to run the query.

4

Fetch and display user names

Use a while loop with $row = $result->fetch_assoc() to print each user's name with echo $row['name'] . "\n";. Then close the connection with $conn->close();.

PHP

query($sql);
// Loop through $result and print each user's name
// Close the connection
// Your code here

Hint

Use fetch_assoc() inside a while loop to get each row as an associative array.

Practice

1. What does the query method in PHP do when used with a database connection?

easy

A. It executes an SQL statement on the database.

B. It creates a new database connection.

C. It formats the SQL query string.

D. It closes the database connection.

Solution

1. Step 1: Understand the purpose of the query method

   The query method is designed to send SQL commands to the database for execution.

2. Step 2: Identify what the method does with the SQL statement

   It runs the SQL command and returns the result, such as data or success status.

3. Final Answer:

   It executes an SQL statement on the database. -> Option A

4. Quick Check:

   query method = executes SQL [OK]

Hint: query method runs SQL commands on the database [OK]

Common Mistakes:

• Thinking query creates or closes connections
• Confusing query with preparing statements
• Assuming query formats SQL strings

2. Which of the following is the correct syntax to execute a SELECT query using the query method on a PDO object named $pdo?

easy

A. $pdo->query = 'SELECT * FROM users';

B. $pdo->execute('SELECT * FROM users');

C. query($pdo, 'SELECT * FROM users');

D. $pdo->query('SELECT * FROM users');

Solution

1. Step 1: Recall the method call syntax for PDO query

   The query method is called on the PDO object using arrow notation with the SQL string as argument.

2. Step 2: Check each option for correct syntax

   $pdo->query('SELECT * FROM users'); correctly calls $pdo->query('SELECT * FROM users');. Others misuse assignment, function call, or wrong method.

3. Final Answer:

   $pdo->query('SELECT * FROM users'); -> Option D

4. Quick Check:

   Correct method call = $pdo->query('SELECT * FROM users'); [OK]

Hint: Use arrow operator and pass SQL string to query() [OK]

Common Mistakes:

• Using assignment instead of method call
• Calling query as a standalone function
• Using execute instead of query for simple queries

3. What will be the output of the following PHP code snippet?

$pdo = new PDO('sqlite::memory:');
$pdo->exec('CREATE TABLE test (id INTEGER)');
$pdo->exec('INSERT INTO test (id) VALUES (1), (2), (3)');
$result = $pdo->query('SELECT COUNT(*) FROM test');
$count = $result->fetchColumn();
echo $count;

medium

A. 3

B. COUNT(*)

C. 1

D. Error

Solution

1. Step 1: Understand the SQL commands executed

   The code creates a table and inserts three rows with ids 1, 2, and 3.

2. Step 2: Analyze the SELECT COUNT(*) query and fetch result

   The query counts rows in the table, which is 3. fetchColumn() returns this count as a string.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Row count = 3 [OK]

Hint: COUNT(*) returns number of rows, fetchColumn gets that value [OK]

Common Mistakes:

• Confusing fetchColumn() with fetch() returning array
• Expecting column name instead of count value
• Assuming query returns error without table

4. Identify the error in the following PHP code that uses the query method:

$pdo = new PDO('mysql:host=localhost;dbname=testdb', 'user', 'pass');
$sql = "INSERT INTO users (name) VALUES ('Alice')";
$result = $pdo->query($sql);
if ($result) {
    echo "Insert successful";
} else {
    echo "Insert failed";
}

medium

A. PDO connection string is wrong.

B. The SQL syntax is incorrect for INSERT.

C. Using query() for INSERT may not return true/false as expected.

D. Missing fetch() after query() call.

Solution

1. Step 1: Check usage of query() with INSERT statements

   The query method can execute INSERT but returns a PDOStatement or false, not a simple true/false.

2. Step 2: Understand how to check success of INSERT

   For INSERT, it's better to use exec() which returns affected rows count or false on failure.

3. Final Answer:

   Using query() for INSERT may not return true/false as expected. -> Option C

4. Quick Check:

   query() returns PDOStatement, not boolean for INSERT [OK]

Hint: Use exec() for INSERT to get affected rows count [OK]

Common Mistakes:

• Expecting query() to return boolean for INSERT
• Confusing query() and exec() methods
• Ignoring return value type of query()

5. You want to retrieve all usernames from a users table and store them in an array using the query method. Which code snippet correctly does this?

hard

A. $usernames = $pdo->query('SELECT username FROM users')->fetch(PDO::FETCH_COLUMN);

B. $stmt = $pdo->query('SELECT username FROM users');
$usernames = $stmt->fetchAll(PDO::FETCH_COLUMN);

C. $usernames = $pdo->query('SELECT username FROM users')->fetchColumn();

D. $usernames = $pdo->query('SELECT username FROM users')->fetchAll();

Solution

1. Step 1: Understand how to fetch all values from a single column

   The fetchAll(PDO::FETCH_COLUMN) fetches all rows from the first column into an array.

2. Step 2: Check each option for correct usage

   $stmt = $pdo->query('SELECT username FROM users'); $usernames = $stmt->fetchAll(PDO::FETCH_COLUMN); correctly calls query(), then fetchAll with FETCH_COLUMN to get usernames array. Others misuse fetchAll or fetchColumn.

3. Final Answer:

   $stmt = $pdo->query('SELECT username FROM users'); $usernames = $stmt->fetchAll(PDO::FETCH_COLUMN); -> Option B

4. Quick Check:

   fetchAll(PDO::FETCH_COLUMN) returns array of column values [OK]

Hint: Use fetchAll(PDO::FETCH_COLUMN) to get single column array [OK]

Common Mistakes:

• Using fetchColumn() to get all rows instead of one
• Not storing query result before fetching
• Passing wrong fetch mode to fetch()