另一个有关的有用方法
declare 
i integer
select nvl(sum(t.dwxhl),1) into i from tb_jhde t where zydm=-1
这样就可以把获得的合计值存储到变量i中，如果查询的值为null就把它的值设置为默认的1 。

GREATEST
    
功能: 集合中的最大值
思路: 确定第一个参数是什么类型,之后所有的参数全部转换成这种类型再比较.

注意:
(1) 参数只能是集合,不能是子查询.
(2) 并且只选择第一个字符最大的.

函数:
FUNCTION GREATEST (pattern NUMBER) return NUMBER;
pragma BUILTIN('GREATEST',12,240,240);-- This is special cased in PH2 -- Pj

function GREATEST (pattern VARCHAR2 CHARACTER SET ANY_CS)
return VARCHAR2 CHARACTER SET pattern%CHARSET;   
pragma BUILTIN('GREATEST',12,240,240);-- This is special cased in PH2 -- Pj

function GREATEST (pattern DATE) return DATE;
pragma BUILTIN('GREATEST',12,240,240);-- This is special cased in PH2 -- Pj

例子:
SELECT GREATEST(SELECT gb.set_of_books_id FROM gl_sets_of_books gb)
FROM dual
--ORA-00936: missing expression

SELECT GREATEST(100,1,3,11)FROM dual;
--100

SELECT GREATEST('100',1,3,11)FROM dual;
--3

SELECT GREATEST('a',1,3,11)FROM dual;
--'a'

SELECT GREATEST('abc','bcd','cde','cda')FROM dual;
--'cde'
--这里只是

SELECT GREATEST(100,1,'3',11)FROM dual;
--100

SELECT GREATEST(100,1,3,'a')FROM dual;
--ORA-01722: invalid number
--无效数字

start with……connect by