Where can I find a syntax or language reference for the query language used on http://data.stackexchange.com/?
When creating a query the right side shows the tables and columns available. The functional details of the schema are described and updated here: Database schema documentation
The database is run on an instance of Microsoft Sql Server and therefor that sql dialect should be used, commonly known as T-SQL. The current version is Microsoft SQL Server 2017 (RTM-CU14) (KB4484710).
Here is an example query to get you going and that demonstrates the features for parameters and magic links as well:
-- Score: Score on the post "a number" -- UserId: Your User id "That is the id in url on the site" select u.id as [User Link] , p.id as [Post Link] , c.id as [Comment Link] from comments c inner join posts p on p.id = c.postid inner join users u on p.owneruserid = u.id where c.userid = ##UserId## and p.score > ##Score:int?2##