Sqlite unrecognized token: "'''"

I met problems while using sqlite3 in python.

def getEntryId(self, table, field, value, createNew=True):
    cur=self.con.execute("select rowid from %s where %s = '%s'" % (table, field, value))
    res=cur.fetchone()
    if res==None:
        cur=self.con.execute("insert into %s (%s) values('%s') " % (table, field, value))
        return cur.lastrowid
    else:
        return res[0]

However, I met this: OperationalError: unrecognized token: "'''". It seems that my 2nd line of codes is incorrect. I can not figure out why, so I do the same thing:

cu.execute("select rowid from urllist where %s = '%s'" % ('url', 'yes'))

It came out without an error. Why? How could I fix it?