sqlite3 数据操作 查询

 打开数据库
    sqlite3 * db = [Database openDB];
    
     数据库的操作指针 stmt:statement
    sqlite3_stmt * stmt = nil;
    
    参数1 , 数据库指针 ,
     参数2 SQL语句
     参数3 SQL语句的长度 (-1 代表的是无限长 , 自动匹配长度)
     参数4 返回数据库操作指针
    参数5 为未来做准备的 , 预留参数 , 一般写成NULL
    
     验证SQL语句的正确性
   int result = sqlite3_prepare_v2(db, "select * from Student", -1, &stmt, NULL);
    
    NSMutableArray * studentArray = [NSMutableArray array];
    
    判断SQL语句的结果
    if (result == SQLITE_OK)
    {
      
        while (sqlite3_step(stmt) == SQLITE_ROW)   存在一行数据
        {
           列数从零开始
            int ID = sqlite3_column_int(stmt, 0);
            
          const unsigned char * name =  sqlite3_column_text(stmt, 1);
            const unsigned char * gender = sqlite3_column_text(stmt, 2);
            int age = sqlite3_column_int(stmt, 3);
            
    blob 类型的获取
             1. 获取长度
            int length = sqlite3_column_bytes(stmt, 4);
             2. 获取数据
            const void * photo = sqlite3_column_blob(stmt, 4);
            
             3.转换成NSData
            
            NSData * photoData = [NSData dataWithBytes:photo length:length];
             4.转成 UIImage
            
            UIImage * p_w_picpath =  [UIImage p_w_picpathWithData:photoData];
            
            
       封装Student模型
            Student * student = [[Student alloc]init];
            student.ID = ID;
            student.name = [NSString stringWithUTF8String:(const char*)name];
            student.age = age;
            student.gender = [NSString stringWithUTF8String:(const char *)gender];
            student.photo = p_w_picpath;
            
             添加到数组中
            [studentArray addObject:student];
        }
    }
    
     释放stmt指针
    sqlite3_finalize(stmt);
    
     关闭数据库
    
    [Database closeDB];