jamiechong

Export 2 Multisite exports your standalone database SQL for import into a WP multisite installation. It handles duplicate users (based on email) and will copy over all site settings including Widgets and Menus. You still need to copy the files for your themes, plugins and uploads.
<ul>
<li>This plugin has been minimally tested, but has been used to successfully migrate a handful of standalone sites to a multisite installation.</li>
<li>There is a good possibility it won’t work for sites that make use of a multitude of plugins – especially plugins that have their own database tables.</li>
</ul>
Installation
<ol>
<li>Create a new site on the Multisite.</li>
<li>Copy the plugins and themes from this standalone site to the Multisite.</li>
<li>Copy the files from wp-content/uploads/ on this site to wp-content/uploads/sites// on the multisite.</li>
<li>Backup the database on the Multisite.</li>
<li>Provide the Site ID of the Multisite. This can be found by hovering the mouse over the new site on Network Admin->Sites.</li>
<li>Provide the New URL. Example: //mysite.my-wp-multisite.com</li>
<li>Provide the New file path (The file path of the Multisite installation).</li>
<li>Export the database dump.</li>
<li>Import the SQL into your Multisite installation.</li>
</ol>
<ul>
<li>Important: The exported SQL will DROP the tables that were created for the Site ID you provided. So only perform this migration on new empty Sites.</li>
<li>Note that the you may need to review the exported SQL to ensure that plugin database tables have been exported correctly.</li>
<li>The site admin (username and password) is the user you created while creating the new Site, NOT the site admin of this standalone site.</li>
<li>The guts of this export plugin were inspired by <a href="https://iandunn.name/comprehensive-wordpress-multisite-migrations/" rel="nofollow ugc">this blog post</a></li>
<li>The code for this plugin has been modified from the <a href="https://wordpress.org/plugins/wp-migrate-db" rel="ugc">WP Migrate DB</a> plugin, but should not conflict with it in any way.</li>
</ul>

Export 2 Multisite

Exports your standalone database SQL for import into a WP Multisite installation.

Olar Marius

Justin Watt

Instead of needing a tool like <a href="http://www.phpmyadmin.net/home_page/index.php" rel="nofollow ugc">phpMyAdmin</a> 
or the mysql command line client to view and modify your WordPress database, 
the SQL Executioner allows you to run arbitrary SQL queries against your 
WordPress database from within the Admin. In many cases this allows you to bypass 
the inherent limitations of the WordPress Admin interface, and use the full expressive 
power of SQL to analyze and update your blog’s database.
To use simply install and visit the Tools > SQL Executioner page.
If you’re interested in contributing to the code behind this plugin, it’s also hosted on GitHub: 
https://github.com/justincwatt/wp-sql-executioner

SQL Executioner

This plugin seems to me simple but very useful.

Simple and efficient

I use this plugin on a daily basis to run quick queries in the database while developing. Very very helpful!!!

Amazing Tool

Execute arbitrary SQL queries against your WordPress database from the Admin.

Export 2 Multisite vs SQL Executioner