oracle中，获得表中所有字段都为null的个数

oracle中，取得表中所有字段都为null的个数
当表的字段个数很多的时候，取得所有字段都是null的个数
字段很少的时候,可以这么写：

DECLARE    TYPE myref IS REF CURSOR;    cur     myref;    sqlStr  VARCHAR2(200);    tb_name VARCHAR2(20);    TYPE arrays IS TABLE OF VARCHAR2(20) INDEX BY BINARY_INTEGER;    column_array arrays;    null_num     INTEGER;BEGIN    tb_name := 'TB';    sqlStr  := 'SELECT column_name FROM user_tab_columns WHERE table_name = ''' ||               tb_name || '''';    OPEN cur FOR sqlStr;    FETCH cur BULK COLLECT        INTO column_array;    CLOSE cur;    sqlStr := 'SELECT COUNT(*) FROM ' || tb_name || ' WHERE 1=1';    FOR i IN column_array.FIRST .. column_array.LAST LOOP        sqlStr := sqlStr || ' AND ' || column_array(i) || ' IS NULL ';    END LOOP;    EXECUTE IMMEDIATE sqlStr        INTO null_num;    dbms_output.put_line(null_num);END;