Creating Multiple Subdomain DB and Integrate to Main Website??

Hello developers, I have created multiple sub domains and their separate databases with full database users privilege for each one. Now i want to integrate all sub domains and Database with main website. How??

 

Thanks for reply ,

In my main domain want to add custom search engine and in subdomains want add QA script, a wordpress blog.

Actually I created all sub domains under main website directory and also created separate database with users full privilege.
Now want to integrate them all for easy to use for users. Means users can access all sub domains under one account.

And for database they used separately while storing and fetching information from server.
eg :

mydomain.com domain database dbusermain
subdomain1.mydomain.com subdomain database1 dbuser1
subdomain2.mydomain.com subdomain database2 dbuser2

 

Want to integrate different databases those are created with separate database users full privilege. eg, storing and fetching information from sub1.domain.com, sub2.domain.com with www.domain.com under one account.

I have subdomain QA.domain.com and blog.domain.com, in main directory that is used for www.domain.com used only for custom search engine with registration and login. if someone login to main domain so users can also access other subdomains.