To use GraphQL in WordPress you have to install the wp-graphql plugin.

To install this plugin You have to download it from GitHub: download wp-graphql

If you don’t know how to install a WordPress plugin manually follow this tutorial

Install a Plugin using the WordPress Admin Plugin Upload.

After Activating this plugin you can visit https://yoursitename.com/graphql to access the GraphQL end-points. Here you can do all the queries and mutation.

If you want to access the GraphQL Playground you have to install this plugin: download WP-GraphiQL 

sources:

wp-graphql github

WP-GraphiQL GitHub

If you want to read things in very detail: read here

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