How to Create Dynamic Queries with wpdb in a WordPress Plugin

Posted on 19th June 2023

Introduction

The WordPress Database abstraction class, wpdb, is one of the most useful tools for developing WordPress plugins. It can be used to interact with the WordPress database in a variety of ways, including creating, updating, and deleting data. In addition, wpdb can be used to run arbitrary SQL queries against the WordPress database.

One of the most powerful features of wpdb is its ability to dynamically generate SQL queries based on user input. This can be extremely useful for plugins that need to perform complex database operations based on user input. In this article, we’ll discuss how to use wpdb to dynamically generate SQL queries in a WordPress plugin.

Creating a Plugin

First, let’s create a plugin that will use wpdb to dynamically generate SQL queries. We’ll start by creating a file named wp-content/plugins/my-plugin/my-plugin.php and adding the following plugin header:

<?php
/*
Plugin Name: My Plugin
Plugin URI: https://example.com/my-plugin
Description: A plugin that demonstrates how to use wpdb to dynamically generate SQL queries.
Version: 1.0
Author: John Doe
Author URI: https://example.com
*/

Next, we’ll need to include the wp-load.php file, which will give us access to the WordPress core functions. We’ll do this by adding the following line of code to our plugin:

require_once( ABSPATH . 'wp-load.php' );

Now that our plugin is set up, we can start using wpdb to dynamically generate SQL queries.

Using wpdb to Dynamically Generate SQL Queries

There are two primary ways to use wpdb to dynamically generate SQL queries:

The prepare() method
The query() method

Both of these methods accept a variable number of arguments. The first argument is always the SQL query itself. The remaining arguments are values that will be substituted into the SQL query in place of the %s placeholder.

For example, consider the following SQL query:

SELECT * FROM wp_posts WHERE post_type = %s AND post_status = %s

This query has two placeholders: %s. We can use the prepare() or query() methods to dynamically generate this query with specific values for the post_type and post_status columns. For example, the following code would generate a query for all posts of the type post that have the status publish:

$sql = $wpdb->prepare( "SELECT * FROM wp_posts WHERE post_type = %s AND post_status = %s", 'post', 'publish' );

Or, we could use the query() method:

$sql = $wpdb->query( "SELECT * FROM wp_posts WHERE post_type = %s AND post_status = %s", 'post', 'publish' );

Either of these methods would generate the following SQL query:

SELECT * FROM wp_posts WHERE post_type = 'post' AND post_status = 'publish'

Once we have our SQL query, we can use the $wpdb->get_results() method to execute the query and retrieve the results:

$results = $wpdb->get_results( $sql );

The $results variable will now contain an array of objects, each of which represents a row from the wp_posts table. We can loop through this array and access the data for each post:

foreach ( $results as $result ) {
	// do something with $result
}

Conclusion

In this article, we’ve discussed how to use wpdb to dynamically generate SQL queries in a WordPress plugin. We’ve also seen how to use the prepare() and query() methods to substitute values into a SQL query, and how to use the $wpdb->get_results() method to execute a SQL query and retrieve the results. By using these methods, we can write WordPress plugins that perform complex database operations based on user input.

In the previous article, we saw how to use the $wpdb global object to run select, insert, update and delete queries on our WordPress database. In this article, we will see how to use $wpdb to create dynamic queries.

We will start by looking at how to create a simple select query. To do this, we will use the $wpdb->get_results() function. This function takes two parameters: the SQL query and an optional array of parameters.

$wpdb->get_results( $sql, $params );

The SQL query can be a string or an array. If it is a string, it will be passed to the database verbatim. If it is an array, it will be converted to a string before being passed to the database.

The optional array of parameters can be used to substitute placeholders in the SQL query with actual values. Placeholders are denoted by a percent sign (%) followed by a number. For example, %1, %2, %3, etc.

The array of parameters must be an associative array, with the keys being the placeholder numbers and the values being the actual values to substitute.

Here is an example of how to use $wpdb->get_results() to run a select query:

$sql = “SELECT * FROM {$wpdb->prefix}options WHERE option_name = %s”; $params = array( ‘my_option_name’ ); $result = $wpdb->get_results( $sql, $params );

In the example above, we are selecting all data from the WordPress options table where the option_name is equal to ‘my_option_name’.

We are using a placeholder ( %s ) in the SQL query and substituting it with the actual value ‘my_option_name’ in the $params array.

The $wpdb->get_results() function returns an array of stdClass objects, one for each row returned by the query.

If we only want to return a single row, we can use the $wpdb->get_row() function. This function behaves exactly the same as $wpdb->get_results() , except it only returns a single row (as an stdClass object) or NULL if no row is found.

Here is an example of how to use $wpdb->get_row() :

$sql = “SELECT * FROM {$wpdb->prefix}options WHERE option_name = %s”; $params = array( ‘my_option_name’ ); $result = $wpdb->get_row( $sql, $params );

If we only want to return a single value from a single row, we can use the $wpdb->get_var() function. This function behaves exactly the same as $wpdb->get_row() , except it only returns a single value (as a string, int, float, etc.) or NULL if no row is found.

Here is an example of how to use $wpdb->get_var() :

$sql = “SELECT option_value FROM {$wpdb->prefix}options WHERE option_name = %s”; $params = array( ‘my_option_name’ ); $result = $wpdb->get_var( $sql, $params );

If we want to run an insert, update or delete query, we can use the $wpdb->query() function. This function takes two parameters: the SQL query and an optional array of parameters.

The SQL query can be a string or an array. If it is a string, it will be passed to the database verbatim. If it is an array, it will be converted to a string before being passed to the database.

The array of parameters must be an associative array, with the keys being the placeholder numbers and the values being the actual values to substitute.

Here is an example of how to use $wpdb->query() to run an insert query:

$sql = “INSERT INTO {$wpdb->prefix}options (option_name, option_value) VALUES (%s, %s)”; $params = array( ‘my_option_name’, ‘my_option_value’ ); $result = $wpdb->query( $sql, $params );

In the example above, we are inserting a new row into the WordPress options table. We are using placeholders ( %s ) in the SQL query and substituting them with the actual values ‘my_option_name’ and ‘my_option_value’ in the $params array.

The $wpdb->query() function returns the number of rows affected by the query (an int).

If we want to get the ID of the last row inserted into the database, we can use the $wpdb->insert_id property. This property will return the ID of the last row inserted into the database by the $wpdb->query() function (as an int).

Here is an example of how to use $wpdb->insert_id :

$sql = “INSERT INTO {$wpdb->prefix}options (option_name, option_value) VALUES (%s, %s)”; $params = array( ‘my_option_name’, ‘my_option_value’ ); $result = $wpdb->query( $sql, $params ); $insert_id = $wpdb->insert_id;

The $wpdb->query() function returns the number of rows affected by the query (an int). The $wpdb->insert_id property returns the ID of the last row inserted into the database (as an int).

If we want to run a query and get the number of rows returned, we can use the $wpdb->num_rows property. This property will return the number of rows returned by the last query run by the $wpdb->query() function (as an int).

Here is an example of how to use $wpdb->num_rows :

$sql = “SELECT * FROM {$wpdb->prefix}options WHERE option_name = %s”; $params = array( ‘my_option_name’ ); $result = $wpdb->query( $sql, $params ); $num_rows = $wpdb->num_rows;

In the example above, we are selecting all data from the WordPress options table where the option_name is equal to ‘my_option_name’.

We are using placeholders ( %s ) in the SQL query and substituting them with the actual value ‘my_option_name’ in the $params array.

The $wpdb->query() function returns the number of rows affected by the query (an int). The $wpdb->num_rows property returns the number of rows returned by the query (as an int).

If we want to run a query and get the number of rows affected, we can use the $wpdb->rows_affected property. This property will return the number of rows affected by the last query run by the $wpdb->query() function (as an int).

Here is an example of how to use $wpdb->rows_affected :

$sql = “DELETE FROM {$wpdb->prefix}options WHERE option_name = %s”; $params = array( ‘my_option_name’ ); $result = $wpdb->query( $sql, $params ); $rows_affected = $wpdb->rows_affected;

In the example above, we are deleting all data from the WordPress options table where the option_name is equal to ‘my_option_name’.

We are using placeholders ( %s ) in the SQL query and substituting them with the actual value ‘my_option_name’ in the $params array.

The $wpdb->query() function returns the number of rows affected by the query (an int). The $wpdb->rows_affected property returns the number of rows affected by the query (as an int).

If we want to run a query and get the last error that occurred, we can use the $wpdb->last_error property. This property will return the last error that occurred while running a query (as a string).

Here is an example of how to use $wpdb->last_error :

$sql = “SELECT * FROM {$wpdb->prefix}options WHERE option_name = %s”; $params = array( ‘my_option_name’ ); $result = $wpdb->query( $sql, $params ); if ( $wpdb->last_error ) { // do something with the error }

In the example above, we are selecting all data from the WordPress options table where the option

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