Syntax options:

    CONSTANT | UPDATABLE

    CONSTANT changes the variable into a declared constant that
    cannot be updated. If you specify CONSTANT, you must also have
    specified the DEFAULT clause to ensure the variable has a value.
    CONSTANT also indicates that the variable cannot be used as
    the target of an assignment or be passed as an expression to a
    procedure's INOUT or OUT parameter.

    UPDATABLE is the default and allows the variable to be modified.
    An update of a variable can occur due to a SET assignment, an
    INTO assignment (as part of an INSERT ... RETURNING, UPDATE ...
    RETURNING, or SELECT statement), or as a procedure's OUT or INOUT
    parameter on a CALL statement.