How to Use wpdb to Retrieve Data Based on Custom Conditions in WordPress Plugin
Posted on 19th June 2023
Introduction
In WordPress plugin development, we often need to retrieve data from the database based on custom conditions. For example, we may need to retrieve all the posts that belong to a certain category or author.
In this article, we will show you how to use wpdb class to retrieve data from the WordPress database based on custom conditions.
What is wpdb Class?
wpdb class is the WordPress class that is used to interact with the WordPress database. It is located in the wp-includes/wp-db.php file.
How to Use wpdb to Retrieve Data?
Before we can use wpdb class to retrieve data, we need to first initialize it with the following code:
$wpdb = new wpdb( ‘username’, ‘password’, ‘database_name’, ‘localhost’ );
Replace username, password, database_name, and localhost with your own database credentials.
Once the wpdb class is initialized, we can use it to run SQL queries on the WordPress database.
For example, the following code will retrieve all the post titles from the WordPress database:
$sql = “SELECT post_title FROM wp_posts”;
$results = $wpdb->get_results( $sql );
foreach ( $results as $result ) {
echo $result->post_title;
}
The get_results() method will return an array of objects. Each object will contain the data for one row from the database.
If you only need to retrieve one row of data, you can use the get_row() method. For example, the following code will retrieve the post title and post content for the post with ID 1:
$sql = “SELECT post_title, post_content FROM wp_posts WHERE ID = 1”;
$row = $wpdb->get_row( $sql );
echo $row->post_title;
echo $row->post_content;
If you only need to retrieve one column of data, you can use the get_var() method. For example, the following code will retrieve the post title for the post with ID 1:
$sql = “SELECT post_title FROM wp_posts WHERE ID = 1”;
$title = $wpdb->get_var( $sql );
echo $title;
How to Retrieve Data Based on Custom Conditions?
In the examples above, we have shown you how to retrieve data from the WordPress database without any conditions.
If you want to retrieve data based on custom conditions, you need to use the prepare() method.
The prepare() method takes two parameters. The first parameter is the SQL query with placeholders (i.e. %s or %d) for the data that will be passed as the second parameter.
For example, the following code will retrieve all the posts that belong to the category with ID 1:
$sql = “SELECT * FROM wp_posts WHERE post_category = %d”;
$results = $wpdb->get_results( $wpdb->prepare( $sql, 1 ) );
foreach ( $results as $result ) {
echo $result->post_title;
}
In the code above, we are using %d as the placeholder for the category ID.
If you want to use more than one placeholder, you need to use an array for the second parameter. For example, the following code will retrieve all the posts that belong to the category with ID 1 and have the status of published:
$sql = “SELECT * FROM wp_posts WHERE post_category = %d AND post_status = ‘%s'”;
$results = $wpdb->get_results( $wpdb->prepare( $sql, 1, ‘publish’ ) );
foreach ( $results as $result ) {
echo $result->post_title;
}
As you can see, we are using %s as the placeholder for the post status.
Conclusion
In this article, we have shown you how to use wpdb class to retrieve data from the WordPress database based on custom conditions.
If you have any questions, please let us know in the comments.
The $wpdb global object can be used to run direct SQL queries on the WordPress database. However, it’s important to use this functionality sparingly, as it bypasses the security provided by the WordPress API, which can lead to vulnerabilities in your code. In most cases, it’s better to use the WordPress API functions to interact with the database.
However, there are some occasions where using $wpdb is the best option. For example, if you need to retrieve data based on custom conditions that are not possible using the WordPress API.
In this article, we’ll show you how to use $wpdb to retrieve data based on custom conditions in a WordPress plugin.
We’ll assume that you have a basic understanding of how to develop WordPress plugins. If you’re not familiar with plugin development, we recommend checking out our guide on how to create a WordPress plugin.
To use $wpdb in a WordPress plugin, you first need to include the wp-db.php file. This can be done by using the require_once function:
require_once( ABSPATH . ‘wp-admin/includes/upgrade.php’ );
Once you have included the wp-db.php file, you can access the $wpdb global object anywhere in your plugin.
One of the most common ways to use $wpdb is to retrieve data from the WordPress database. This can be done using the $wpdb->get_results() function.
This function accepts an SQL query as its first parameter. This query can be a string or an array.
If you pass a string to the $wpdb->get_results() function, it will be interpreted as an SQL query. For example, the following code will return all posts from the WordPress database:
$results = $wpdb->get_results( “SELECT * FROM {$wpdb->posts}” );
If you pass an array to the $wpdb->get_results() function, it will be interpreted as a prepared SQL statement. This is useful if you need to pass variables to the SQL query.
For example, the following code will return all posts with the ID of 1:
$results = $wpdb->get_results(
“SELECT * FROM {$wpdb->posts} WHERE ID = %d”,
array(
1,
)
);
It’s also possible to retrieve a single row from the database using the $wpdb->get_row() function. This function accepts the same parameters as the $wpdb->get_results() function.
For example, the following code will return the post with the ID of 1:
$post = $wpdb->get_row(
“SELECT * FROM {$wpdb->posts} WHERE ID = %d”,
array(
1,
)
);
If you need to retrieve a specific column from the database, you can use the $wpdb->get_var() function. This function accepts an SQL query as its first parameter. It also accepts an optional second parameter, which is the column number to return.
If no column number is specified, the $wpdb->get_var() function will return the first column of the first row.
For example, the following code will return the post_title column of the post with the ID of 1:
$title = $wpdb->get_var(
“SELECT post_title FROM {$wpdb->posts} WHERE ID = %d”,
array(
1,
)
);
It’s also possible to insert, update, and delete data using $wpdb. However, we recommend using the WordPress API functions for these operations, as they provide a more secure and reliable way to interact with the database.
If you need to run a custom SQL query that is not possible using the WordPress API, then $wpdb is the best option. However, you should use this functionality sparingly, as it bypasses the security provided by the WordPress API. In most cases, it’s better to use the WordPress API functions to interact with the database.
