Executing queries with query method in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the query method do in PHP's mysqli?

The query method sends an SQL query to the database and returns the result set or false if the query fails.

beginner

How do you check if a query was successful in PHP?

You check if the result of $mysqli->query() is not false. If it is false, the query failed.

beginner

What type of value does query return for a SELECT statement?

It returns a mysqli_result object which you can use to fetch rows from the result set.

beginner

What does query return for an INSERT, UPDATE, or DELETE statement?

It returns true if the query was successful, or false if it failed.

beginner

How do you fetch rows from the result of a query?

Use methods like fetch_assoc() on the mysqli_result object to get rows as associative arrays.

What does $mysqli->query() return when a SELECT query is successful?

A True

B False

C A mysqli_result object

D An integer

For SELECT queries, query returns a mysqli_result object to fetch rows.

What does $mysqli->query() return when an INSERT query is successful?

A A mysqli_result object

B True

C False

D Null

For INSERT, UPDATE, DELETE queries, query returns true on success.

How can you check if a query failed?

A Check if <code>query</code> returns <code>false</code>

B Check if <code>query</code> returns <code>true</code>

C Check if <code>query</code> returns a string

D Check if <code>query</code> returns an integer

If query returns false, the query failed.

Which method do you use to get rows from a mysqli_result object?

A execute()

B query()

C connect()

D fetch_assoc()

Use fetch_assoc() to get rows as associative arrays.

What happens if you run query with a wrong SQL syntax?

A It returns false

B It returns true

C It returns an empty result

D It throws an exception automatically

If the SQL syntax is wrong, query returns false.

Explain how to use the query method to run a SELECT statement and get results.

Think about sending the query, checking success, and reading rows.

Describe what the query method returns for different types of SQL statements.

Consider the difference between queries that return data and those that just change data.

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()