NEXTVAL and PREVVAL expressions can be specified in the following
places:
– select-statement or SELECT INTO statement (within the select-clause,
provided that the statement does not contain a DISTINCT keyword, a
GROUP BY clause, an ORDER BY clause, a UNION keyword, an
INTERSECT keyword, or EXCEPT keyword)
– INSERT statement (within a VALUES clause)
– INSERT statement (within the select-clause of the fullselect)
– UPDATE statement (within the SET clause (either a searched or a
positioned UPDATE statement), except that NEXTVAL cannot be
specified in the select-clause of the fullselect of an expression in the SET
clause)
– SET Variable statement (except within the select-clause of the fullselect of
an expression; a NEXTVAL expression can be specified in a trigger, but a
PREVVAL expression cannot)
– VALUES INTO statement (within the select-clause of the fullselect of an
expression)
– CREATE PROCEDURE statement (within the routine-body of an SQL
procedure)
– CREATE TRIGGER statement within the triggered-action (a NEXTVAL
expression may be specified, but a PREVVAL expression cannot)

NEXTVAL and PREVVAL expressions cannot be specified (SQLSTATE
428F9) in the following places:
– join condition of a full outer join
– DEFAULT value for a column in a CREATE or ALTER TABLE statement
– generated column definition in a CREATE OR ALTER TABLE statement
– summary table definition in a CREATE TABLE or ALTER TABLE
statement
– condition of a CHECK constraint
– CREATE TRIGGER statement (a NEXTVAL expression may be specified,
but a PREVVAL expression cannot)
– CREATE VIEW statement
– CREATE METHOD statement
– CREATE FUNCTION statement

In addition, a NEXTVAL expression cannot be specified (SQLSTATE 428F9)
in the following places:
– CASE expression
– parameter list of an aggregate function
– subquery in a context other than those explicitly allowed above
– SELECT statement for which the outer SELECT contains a DISTINCT
operator
– join condition of a join
– SELECT statement for which the outer SELECT contains a GROUP BY
clause
– SELECT statement for which the outer SELECT is combined with
another SELECT statement using the UNION, INTERSECT, or EXCEPT
set operator
– nested table expression
– parameter list of a table function
– WHERE clause of the outer-most SELECT statement, or a DELETE or
UPDATE statement
– ORDER BY clause of the outer-most SELECT statement
– select-clause of the fullselect of an expression, in the SET clause of an
UPDATE statement
– IF, WHILE, DO ... UNTIL, or CASE statement in an SQL routine