python查询数据库结果返回

python操作数据库，在查询中遇到几个令人恶心的问题，语句都没问题，就是报错。
sql_user_item_operationsdays = sql_user_item_operationsdays = 'select sum(opernum) as numbers from %s where user_id=%d and behavior_type=%d and diffdays>%d'

usertype = [2]#[1,2,3,4]

dayList = [26]#[26, 22, 15, 8, 0]

for type_line in usertype:

for day_line in dayList:

sql = sql_user_item_operationsdays % (filename, user_id, type_line, day_line)

print sql

sqlnum = self.cur.execute(sql)

print 'sqlnum is %d' % sqlnum

result = self.cur.fetchone()

print 'result is ' + result

temp = self.result_fetch(sqlnum, result)

print 'temp is %d' % temp

user_featuresList.append( float(temp))

上面代码是我写的一个提取特征部分代码，最终发现原来是cur.execute()执行sql语句返回结果的问题；一般情况sqlnum=cur.execute(sql)，
sql语句若查不到结果，返回0；但是上面怎么就不返回0，而是返回1，我再三验证，原来还真是1，但是result=cur.fetchone()返回的却是((None,),)，
最终发现select sum()这里返回时，若查不到会返回1，但是result元组中存的是NontType类型变量None，因此程序一直报错。

conn = mysqldb.connect(host='localhost', user='root',passwd='',port=3306)

cur = conn.cursor()

conn.select_db('tianchi_420')

sql1 = 'select sum(opernum) from user_item_operations29days where user_id=100509623 and behavior_type=2 and diffdays>26';

print sql1

sqlnum = cur.execute(sql1)

print sqlnum

result = cur.fetchall()

print result

print '----------------------------'

sql1 = 'select * from user_item_operations29days where user_id=100509623 and behavior_type=2 and diffdays>26';

print sql1

sqlnum = cur.execute(sql1)

print sqlnum

result = cur.fetchall()

print result

print '----------------------------'

sql1 = 'select count(*) from user_item_operations29days where user_id=100509623 and behavior_type=2 and diffdays>26';

print sql1

sqlnum = cur.execute(sql1)

print sqlnum

result = cur.fetchall()

print result

print '-------------------------------'

结果如下：
select sum(opernum) from user_item_operations29days where user_id=100509623 and behavior_type=2 and diffdays>26

1

((None,),)

----------------------------

select * from user_item_operations29days where user_id=100509623 and behavior_type=2 and diffdays>26

0

()

----------------------------

select count(*) from user_item_operations29days where user_id=100509623 and behavior_type=2 and diffdays>26

1

((0L,),)

注意三者返回的都有区别；