PL/SQL 找到某列都为空的列名

DECLARE
CURSOR temp IS SELECT COLUMN_NAME FROM ALL_TAB_COLUMNS WHERE TABLE_NAME=Upper('xxx');
v_num NUMBER;
BEGIN
FOR i IN temp
LOOP
execute immediate 'SELECT Count(*) FROM xxx WHERE '||i.COLUMN_NAME||' IS NOT null' INTO v_num;
IF v_num=0 THEN
Dbms_Output.put_line('==columns that have no data=='||i.COLUMN_NAME);
END IF;
NULL;
END LOOP;
END;

xx为表名