NSString

stringWithUTF8String:

const char *school = (char *)sqlite3_column_text(statement, 18);
    if (school) {
        person.school = [NSString stringWithUTF8String:school];
    }
sqlite允许插入空值，但是stringWithUTF8String这个方法处理不了空值，所以必须判定school是否为空！如果传一个空值进去，会发生崩溃

有时这个方法处会崩溃，原因可能在对象被autorelease了