ADQL defines just one statement, the SELECT statement, which lets you write down expressions of relational algebra. Roughly, it looks like this:
SELECT [TOP setLimit] selectList FROM fromClause [WHERE conditions] [GROUP BY columns] [ORDER BY columns]
In reality, there are yet a few more things you can write, but what’s shown covers most things you’ll want to do. The real magic is in selectList, fromClause (in particular), and conditions.
setLimit: just an integer giving how many rows you want returned.
SELECT TOP 5 * FROM gaiadr1.tgas_source
SELECT TOP 10 * FROM gaiadr1.tgas_source