Direct database queries (wpdb) in Wordpress - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What does this wpdb query return?

Consider this WordPress wpdb query:

global $wpdb;
$result = $wpdb->get_var("SELECT COUNT(*) FROM {$wpdb->prefix}posts WHERE post_status = 'publish'");
echo $result;

What will be the output if there are 10 published posts in the database?

Wordpress

global $wpdb;
$result = $wpdb->get_var("SELECT COUNT(*) FROM {$wpdb->prefix}posts WHERE post_status = 'publish'");
echo $result;

A Error: Undefined variable

B Array with 10 elements

C NULL

D 10

💡 Hint

get_var returns a single value from the first column of the first row.

The get_var method returns a single value from the database query. Since the query counts published posts and there are 10, it returns '10' as a string.

📝 Syntax

intermediate

Which wpdb query syntax is correct to safely insert data?

You want to insert a new user with name and email into a custom table using wpdb. Which option uses the correct wpdb method and syntax to safely insert data?

A $wpdb->insert('wp_custom_users', ['name' => $name, 'email' => $email]);

B $wpdb->query("INSERT INTO wp_custom_users (name, email) VALUES ('$name', '$email')");

C $wpdb->insert('wp_custom_users', $name, $email);

D $wpdb->get_results("INSERT INTO wp_custom_users (name, email) VALUES (?, ?)", [$name, $email]);

💡 Hint

Look for the method designed for inserting data with automatic escaping.

The insert() method takes the table name and an associative array of column => value pairs and safely escapes the data. Option A uses this correctly.

🔧 Debug

advanced

Why does this wpdb query cause an error?

Look at this code snippet:

global $wpdb;
$user_id = 5;
$result = $wpdb->get_row("SELECT * FROM {$wpdb->prefix}users WHERE ID = $user_id");
echo $result->user_login;

Why might this cause a PHP error?

Wordpress

global $wpdb;
$user_id = 5;
$result = $wpdb->get_row("SELECT * FROM {$wpdb->prefix}users WHERE ID = $user_id");
echo $result->user_login;

A get_row returns an array, so ->user_login is invalid.

B The query syntax is invalid because variables cannot be inside double quotes.

C $result is NULL if no user with ID 5 exists, so accessing ->user_login causes an error.

D The $wpdb->prefix is undefined causing a fatal error.

💡 Hint

Check what happens if the query returns no rows.

If the user with ID 5 does not exist, get_row returns NULL. Trying to access a property on NULL causes a PHP error.

❓ state_output

advanced

What is the value of $count after this wpdb query?

Given this code:

global $wpdb;
$count = $wpdb->get_var($wpdb->prepare("SELECT COUNT(*) FROM {$wpdb->prefix}posts WHERE post_author = %d", 3));

Assuming the author with ID 3 has 7 posts, what is the value of $count?

Wordpress

global $wpdb;
$count = $wpdb->get_var($wpdb->prepare("SELECT COUNT(*) FROM {$wpdb->prefix}posts WHERE post_author = %d", 3));

A 7

B Array with 7 elements

C Error: prepare method not found

D NULL

💡 Hint

prepare safely inserts the number 3 into the query.

The prepare method replaces %d with 3 safely. The query counts posts by author 3, which is 7, so get_var returns '7'.

🧠 Conceptual

expert

Which wpdb method should you use to get multiple rows as objects?

You want to retrieve all published posts from the database as an array of objects. Which wpdb method is the best choice?

A $wpdb->get_var("SELECT * FROM {$wpdb->prefix}posts WHERE post_status = 'publish'");

B $wpdb->get_results("SELECT * FROM {$wpdb->prefix}posts WHERE post_status = 'publish'");

C $wpdb->get_row("SELECT * FROM {$wpdb->prefix}posts WHERE post_status = 'publish'");

D $wpdb->insert("{$wpdb->prefix}posts", ['post_status' => 'publish']);

💡 Hint

Think about which method returns multiple rows.

get_results returns an array of objects for multiple rows. get_var returns a single value, get_row returns one row, and insert adds data.