Sharepoint Table Name where Web Applications URL are stored?

John and Dave already told you it's not recommended to query SharePoint database however just to answer your question,

In SharePoint Config database there is a table named as dbo.SiteMap which has column ApplicationId which is actually id of all web application's in your farm including CA. You can get list of distinct application id's.

Since you got all applicationId's you can get web application object and then it's url using powershell,

$a=Get-SPWebApplication(“http://xxxxxxx”)
$a.ID

SharePoint 2007: Getting object ID's using Sharepoint Central Administration

Note:

Approach is not recommened.

Simply put: Don't ever query the SharePoint admin or content databases directly in SQL.

There are a number of other ways to get the web applications for the current farm.

1) PowerShell: Get-SPWebApplication

2) In C#:

var webApps = new Collection<SPWebApplication>();
if (SPWebService.AdministrationService.WebApplications != null)
    webApps.AddRange(SPWebService.AdministrationService.WebApplications.Where(p => p.IisSettings.Count > 0));
if (SPWebService.ContentService.WebApplications != null)
    webApps.AddRange(SPWebService.ContentService.WebApplications.Where(p => p.IisSettings.Count > 0));

The AdministrationService is where the Central Admin web app will be and the ContentService is where the rest will be.

Now, as far as your comment on getting communities, you have two ways to do this in 2013:

1) Use the SocialFollowingManager to retrieve all the sites the user is following. See http://msdn.microsoft.com/en-us/library/jj163818.aspx

2) Traverse through every SPWeb of every SPSite of every SPWebApplication to see if the user has permissions to the site.

There is probably an easier way but here's a powershell script that should get you close without having to violate your support agreement with Microsoft.

[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint")
[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.Office.Server.UserProfiles")


Get-SPWebApplication | % {
    $webApp = $_
    $ctx = Get-SPServiceContext $webApp.sites[0]
    $upm = [Microsoft.Office.Server.UserProfiles.UserProfileManager]($ctx) 

    $obj=New-Object System.Object
    $obj | Add-Member -type NoteProperty -name WebAppUrl -value $webApp.Url
    $obj | Add-Member -type NoteProperty -name MySiteUrl -value $upm.MySiteHostUrl

    $obj 
}