decode (expression, search_1, result_1)
如果 expression结果＝search_1结果，则返回result_1，类似 if else
decode (expression, search_1, result_1, search_2, result_2)
decode (expression, search_1, result_1, search_2, result_2, ...., search_n, result_n)
decode (expression, search_1, result_1, default)
如果 expression结果＝search_1结果，则返回result_1，否则反回default
decode (expression, search_1, result_1, search_2, result_2, default)
decode (expression, search_1, result_1, search_2, result_2, ...., search_n, result_n, default)
decode函数比较表达式和搜索字，如果匹配，返回结果；如果不匹配，返回default值；如果未定义default值，则返回空值。
如：
select (a/b*100) as percent from t
如果b为0会出现被除数为0
解决：
select decode(b,0,0,a/b*100) as percent from t