How to connect to muliple databases using same queries - c++

Question

I am developing a large scale application in C++. The application also maintains a database (currently I am using MySQL), I use OTL for database connectivity. Now what i want to do is to provide support for using database from multiple vendors. E.g User A use MySQL and user B use PostGres. I think upon implementing it in C++ but didn't come up with any possible solution due to lack of experience.
What I want to achieve, is something like that:
There would be a separate VC Project that deals with database and suppose it contains following files:
DataAccessLayer.cpp //This will main entry point of the project
Product.cpp //This deals with product table
Customer.cpp // This deals with Customer table
Orders.cpp //This deals with Orders table
. . . and many more // I want to have one cpp file per Database table`
And we will use the above project in our code like this
DataAccessLayer oDataAccessLayer;
oDataAccessLayer.Connect(); // This will connect to specified database, it might b some abstract class and have concrete class for each supported DB
oDataAccessLayer.Products.Search(//Some parameters here e.g prod id to b search);//I don't want to write search query again again for each database, This function will execute the query in specific database
oDataAccessLayer.Customers.Add(//Parameter)//Same is the case here I don't want to write ADd query for each supported database
oDataAccessLayer.Disconnect();
I don't want the whole code I just need some sample code or related article to study.

Answer 1

Your question is a bit ambiguous. However, I will base my answer on the perspective that you want your code to access one physical database, which is configurable to MySQL, PostGres, Oracle, etc.
Generate generic SQL statements.
Instead of accessing the MySQL library for inserting records, build a SQL statement and let the connector execute the statement. This reduces the interface to one small point: the connector. I have my Field and Record classes set up to use the Visitor pattern. I create Visitors to build the SQL statements. This enables my database component to be more generic.
Abstract the DB Connector.
Create your own Connector object as a Facade around the DB manufacturer's connector. Make the methods generic, such as passing a string containing SQL text. Next, change your components to require passing of this facade during construction or when accessing the database. Finally, create an instance of this facade to communicate with specific database applications before using any of your components.
Suggestions
I have found that having the Record class contain the table name, I could eliminate the need for a Table class (which models the database table). Appending and loading is handled by my database Manager class. In my recent progress, I implemented functionality for prepared statements and BLOB fields.

Answer 2

Suppositions:
Suppose i have following two tables in my database
Project
Docs
Docs
And currently i am supporting only following two databases:
1) Post Gres
2) Oracle
Implementation:
I implement this logic in following three phases
1) Data Base Adapter: Only this class is exposed to outer world and this contians the Pointer to Each QueryBuilder, these pointer will be initialized in the constructor of this class on the basis of DB type(which is passed as an argument to constructor)
2) DB Connector: This class will be responsible for DB handling (establish connection, disconnect etc.)
3) Query Builder: This section contains a basse class for each table in DB and also contains concrete calss for each each specific database
Sample Code (Synopsis Only)
DBAdapter.h
DBAdapter(enum DBType, string ConnectionString);//constructor
IntializeDBConnectAndQueryBuilder();
Project *pProject;
Docs *pDocs;
//----------------------------------------------------------------------------------------
DBAdapter.cpp
DBAdapter(enum DBType, string ConnectionString)//constructor
{
swithc (DBType)
{
case enPostGres:
pProject = new Project_PostGres();
pDocs = new Docs_PostGres();
break;
};
}
DB Connector Section:
DBConnector.h
virtual Connect()=0; //pure virtual
IsConnected()=0;//pure virtual
Disconnect()=0; //pure virtual
//------------------------------------------------------------------------------------------
DBConnector_PostGres.cpp: DBConnector
Connect()
{ //Implementation specific to postgres goes here}
IsConnected()
{ //Implementation specific to postgres goes here}
Disconnect()
{ //Implementation specific to postgres goes here}
//------------------------------------------------------------------------------------------
DBConnector_OracleGres.cpp: DBConnector
Connect()
{ //Implementation specific to oracle goes here}
IsConnected()
{ //Implementation specific to oracle goes here}
Disconnect()
{ //Implementation specific to oracle goes here}
//------------------------------------------------------------------------------------------
Query Builder Section:
Project.h
virtual int AddProject();
virtual int DeleteProject();
virtual int IsProjectTableExist();
DBConnector *pDBConnector; // This pointer will be initalized in all concrete classes
//------------------------------------------------------------------------------------------
Project.cpp
int AddProject()
{
//Query for adding Project is same for all databases
}
int DeleteProject()
{
//Query for deleting Project is same for all databases
}
//------------------------------------------------------------------------------------------
Project_PostGres.cpp : Project
Project_PostGres() // Constructor
{
pDBConnector = new DBConnector_PostGres();
}
int IsProjectTableExist()
{
//Query specific to postgres goes here
}
Project
//------------------------------------------------------------------------------------------
Project_Oracle.cpp : Project
{
//Query specific to oracle goes here
}
//. . . and so on for other databases
//------------------------------------------------------------------------------------------
//Same number of files with same logic will also be created for Docs Table
------------------------------------------------------------------------------------------