Oracle has a special "virtual" table called dual, which is commonly used to select from when all you want to do is run a function, for example:

select sqrt(25) + power(2,3) - round(4.645432, 1) from dual;
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The SQL in the example, select 1 from dual, is often used to ping the database. It essentially does nothing at all except talk to the database and ask it to return the value 1. I don't think this is standard SQL stuff, but rather an Oracle-specific extension.