When you specify the ALL keyword before a subquery, Pervasive PSQL performs the subquery and uses the result to evaluate the condition in the outer query. If all the rows the subquery returns meet the outer query's condition for a particular row, Pervasive PSQL includes that row in the final result table of the statement.
The following SELECT statement compares the ID column from the Person table to the ID columns in the result table of the subquery: