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Next: 3. Important Changes Up: Introductory Material Previous: 1. Introduction   Contents

Subsections

2. Overview

The Mysql++ API has developed into a very complex and powerful being. With many different ways to accomplish the same task. Unfortunately this means that figuring out how to perform a simple task can be frustrating for new users of my library. In this section we will attempt to provide an overview of the many different components of the library.

Like working with most other SQL API the process for executing queries is the same. 1) You open the connection, 2) You form and execute the queries, 3) You iterate through the result set. It not much different in my C++ API. However there is a lot of extra functionality along each step of the way.

The Main Database Handle

This is a class that handles the connection to the Mysql server. You always need at least one of these objects to do anything. It can either create a separate queries object or directly execute queries. The separate query object is the recommended way as it gives you far more power.

The Query Object

This object is the recommended way of executing queries. It is subclassed from strstream which means you can write to it like any other stream to aid in the formation of queries.

You can also set up Template queries with this class. Template queries are a way of setting up queries with replaceable parameters that you can change throughout your program.

You can also use specialized structures and even the dramatic result sets to aid in creating queries however more on that latter.

The Query object returns an object with information about the success of a query for non-select queries (queries that don't return a result set).

The Result Sets

For queries that return a result set you have essentially two different ways of handling the results: in a dramatic result set, or in a static one.

The Dramatic Result Set

The Dramatic Result set is a result set in which the names of the columns and the type of information of the columns does not need to be determined at compile time. The result set can be completely constant in which the data is returned to you in a constant string link class, semi-constant in which you can modify the data one row at a time, or a truly mutable in which in you can modify the data in any way you like.

The constant result set is a result set that is closely bound to the result set in the C API and is the one that provides the most functionality. With this result set you can find out detailed information about the type of information stored in each of the columns. This is also the fastest because the data does not need to be copied at all.

The semi-constant result set is like the constant result set except you can modify the data one row at a time. The data you modify is actually a copy of the data returned by the server. This means that modifying the data does not change the actual result set at all.

The semi-constant result set is actually the same thing as the constant result set. The only difference is that when you request a row from the result set you specifically declare the row as a mutable one. This means that you can get some rows back as constant rows and others as mutable ones.

The truly mutable result set is a result set similar to the constant one except that the data is truly mutable in the sense that you can change the data in the actual result set. However unlike the first one this result set is not bound to the C API result set. Instead it containers a copy of the data returned by the C API in a two-dimensional vector. Because of this the detailed information about each of the columns is not currently available, only the column names and the C++ type that most closely matches the original SQL type. Also, because it makes a copy of the data returned from the C API, there is a little bit of performance penalty to using this one.

The rows in all the dramatic result sets are very close to an Standard Template Library (STL) random access container. This means that they have an iterator which can be used for STL algorithms. There is even couple of specialized utility function to aid in the use of the result sets in STL algorithms.

The columns in all the dramatic result are also very close to an STL random access container. However, in addition to accessing the columns by there index number you can also access the columns via there field names.

In addition, because both the rows and the columns are STL like containers, you can also treat the result set as a two- dimensional array. For example you can get the 5th item on the 2nd row by simply saying result[2][5]. Because you can also use the field names you can substitute the column number by a field name and say result[2]["price"] to get "price" of the item on the 2nd row, for example.

The actual data that all the dramatic result sets return is stored in a special string like class that has some additional magic too it. The magic is that the column data will automatically convert itself into all of the basic data types as well as some additional types types that are designed to handle mysql types which includes types for handling dates, times, sets, and types with a null value. If there is a problem in the conversion it will either set a warning flag or throw an exception depending on how it is configured. Regarding exceptions, MySQL++ supports two different methods of tracing exceptions. One is by the fixed type (the old one) and one is standard C++ type by the usage of what() method. A choice of methods has to be done in building a library. If configure script is run with -enable-exception option , then new method will be used. If no option is provided, or -disable-exception is used, old MySQL++ exceptions will be enforced.

The drastic result sets can even be used to help form queries with the help of some additional method. There is a method for returns: 1) A comma separated list of the data (for example: 1.5, 10, "Dog, "Brown"), 2) A comma separated list of the field names (for example: age, weight, what, color), and 3) An equal list (for example: age = 1.5 AND weight = 10 AND what = "Dog" AND color = "Brown").

Mutable result sets can be created with out an actual query so that you can take advantage of these methods to aid in inserting data into the database with out having to first create an unnecessary query.

The Static Result Sets

The results from an query can also be stored statically in what we call a specialized SQL structure. These structures are then stored in some STL container such a vector or list, or even a set or multi-set as the the specialized structures can also be made less-than-comparable. Unlike the dramatic result sets it is assumed that the programmer knows what the result set is going to look like. Because of this all the information about the columns, including the names, are lost.

These Specialized Structures are exactly that C++ `structs'. Each member item is stored with a unique name within the structure. You can in no way use STL algorithms or anything else STL to work with the individual elements of the structures. However naturally because these structures are then stored in STL containers you can use STL algorithms on the containers of these structures. The containers represent the rows, and the individual elements of the structure represent the columns. For example you can access the item named "price" on the second row by saying result[2].price. With the dramatic result set you would have probably needed to say result[2]["price"] to accomplish the same result.

If there is a problem in converting from the result set returned by the server to the specialized structures an exception is thrown.

To aid in the creating of queries using these specialized structures, the same query aiding methods are available to use that are available for the dramatic result sets. This includes methods for returning a comma separated list of the data, a comma separated list of the field names, and an equal list.

The Dynamic Fully Mutable Sets

This result set will be implemented when server-side cursors are implemented in MySQL. But, based on so far acquired knowledge and experience, from designing and implementing both MySQL++ and MySQLGUI, a preliminiary layout and design of the most advanced result set so far has been achieved. It's implementation is postponed, however, from the above reasons. This result set will be fully dynamic and dramatic. This result set is fully dynamic in a sense that entire result set is stored in a dynamic C++ container. This container will be only a cache , a dynamic cache, to the entire result set, and will have a default size. This dynamic container will be a window of M rows into an entire result set of N rows, where N is limited by MySQL server , Operating System and File System only. This result set will also be fully dramatic in a sense that the names of the columns and the type of information of the columns will not need to be determined at compile time. But all existing functionality of static , mutable sets will be available in this set too. However as this set will be dramatic, no advance information on result set structure will be necessary, which will thus aleviate need for the usage of specialized macros for the construction of classes. This set will also have methods for updating, deleting and inserting rows in a manner that will be almost identical for use as methods for the existing fully mutable sets.

In addition

In addition to the material mentioned there are also many generic classes that can be used with other programs. Examples of this include a special const string class, a const random access adapter that will make a random access container out of a class with nothing but the size() method and the subscript ([]) operator defined and a generic SQL query class that can be used any SQL C or C++ API.

As from version 1.7, there is a new addtion to the libraries. Several very usefull functions for STL strings can be added, which can be used in any C++ aplication, MySQL++ related or not. Those functions are contained in source files string_util.hh and string_util.cc.


next up previous contents
Next: 3. Important Changes Up: Introductory Material Previous: 1. Introduction   Contents
2001-05-01
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