Null values in collections

A collection cannot contain NULL elements. However, when the collection is a row type, you can insert NULL values for any or all fields of a row type that a collection contains. Suppose you create the following table that has a collection column:

CREATE TABLE tab1 (col1 INT,
        col2 SET(ROW(a INT, b INT) NOT NULL));

The following statements are allowed because only the component fields of the row type specify NULL values:

INSERT INTO tab1 VALUES ( 25,"SET{ROW(NULL, NULL)}");

INSERT INTO tab1 VALUES ( 35,"SET{ROW(4, NULL)}");

INSERT INTO tab1 VALUES ( 45,"SET{ROW(14, NULL), ROW(NULL,5)}");

UPDATE tab1 SET col2 = "SET{ROW(NULL, NULL)}" WHERE col1 = 45;

However, each of the following statements returns an error message because the collection element specifies a NULL value:

INSERT INTO tab1 VALUES ( 45, "SET{NULL)}");

UPDATE tab1 SET col2 = "SET{NULL}" WHERE col1 = 55;