A question from someone that has never used GraphQL: what makes it better than SQL? You don't even need parsers in different languages for SQL, you could just send it straight to the DB. But I assume I'm missing something.
In real life there will usually be a lot more going on to serve a GraphQL query than simply translating it to SQL.
The data you'll send might come from many different sources, including any kind of database, caches, or just constants in your code.
As commented here, permissions over data is a big topic, and if you were accepting raw SQL, you'd have to parse / tokenize it, and somehow validate it before executing it. This is what GraphQL does, but instead of trimming down and restricting the usage of SQL, it defines a language that is similar, close to being a subset of it, with a much simpler syntax.
You probably don't want to do that for security reasons. So either way you need a parser, and at that point, it makes sense to use something that is database agnostic and allows you to model relationships without complex joins or otherwise exposing the internal representation of the data.
http://githubengineering.com/the-github-graphql-api/
It's not really GraphQL vs SQL, it's GraphQL vs RESTful APIs.