A subordinate clause is a type of clause that cannot stand alone.
Subordinate clauses are introduced by subordinating conjunctions.
If the function has been defined as returning the returns all rows in the qualified Cartesian product (i.e., all combined rows that pass its join condition), plus one copy of each row in the left-hand table for which there was no right-hand row that passed the join condition.
This left-hand row is extended to the full width of the joined table by inserting null values for the right-hand columns. Any row that does not satisfy this condition will be eliminated from the output.
the select for update clause: "locks the selected rows so that other users cannot lock or update the rows until you end your transaction.