It doesn’t exist. Sad but true.
In SQL there is a clause that ensures a duplicate free result set is returned by any query you issue e.g. You want to retrieve a duplicate free list of first names from the table clients. The column holds the following values: Alice, Bob, Alice. In Oracle SQL you would use the following query (and it would return the names Alice and Bob each listed once).
SELECT DISTINCT firstname
FROM clients;Unfortunately SOQL doesn’t have this capability although there is an Apex trick that can be employed as a work-around.