what is SWC Agent Database in keychain?

I can confirm that "SWC Agent Database" is in the Keychain on my Mac mini running macOS Mojave.

In addition to leroydouglas' excellent breakdown on this launch agent process:

• The swc daemon (swcd) components can be located at:
  • /System/Library/LaunchAgents/com.apple.swcd.plist
  • /System/Library/Sandbox/Profiles/com.apple.swcd.sb
  • /usr/libexec/swcd

there's a keychain item simply called "SWC Agent Database", and the "Access Control" tab shows to allow application "swcd" to access it. I've never seen this in keychain before

I would suspect there are many things in the Keychain you are not able to identify—is it cause for concern— no.

"SWC Agent Database" — is Shared Web Credentials and is part of the macOS / iOS system files LaunchServices.applicationRegistered

along with swcagent, swcd.

Some factoids:

• SWC is shared web credential.
• swcd is the shared web credential daemon.
• That daemon uses NSURLSession to fetch the apple-app-site-association file.
• That message is generated when NSURLSession delivers the -URLSession:task:willPerformHTTPRedirection:newRequest:completionHandler: delegate callback to swcd.
• When swcd
• gets this delegate callback and issues this log message, it always
• calls the completion handler with nil, causing the body of the
• redirection response to be delivered as the payload of this request.

I would not recommend deleting it.

To help troubleshoot your Mac, I would suggest that you consider using EtreCheck and posting the resultant report for us to review. (Note: The EtreCheck report does NOT contain any personal information.) 

• You can download EtreCheck from here.
• Start EtreCheck from a normal user account. Optionally, you can run it from a user account with Administrator privileges.
• Select a problem from the drop-down menu to enable the "Start EtreCheck" button. Optionally, you can add comments on what issues your Mac is experiencing, especially to aide others with similar Mac issues.
• Click on Start EtreCheck
• Allow the program to run to completion.
• When done, select Report from the left-side window to display it.
• Select the Share Report icon, then select Copy Report.
• Paste the report to your reply post. Hint: Use the Add Text option from the ASC editor's menu bar.

Ref:

• Using EtreCheck - ASC (etresoft)
• How to use the Add Text Feature When Posting Large Amounts of Text, i.e. and Etrecheck Report - ASC (Old Toad)

First off you're not installing anything. EtreCheck does not include an installer. Second it helps us determine the location of any agents or daemons that may be the cause. Finally, it's only a suggestion. If you don't want to try it, fine, then don't. That's your call. We're just trying to help.

You're very welcome! Although I have been around well before System 8, I started with punch cards to run FORTRAN on main frames, then on to Commodore PET > original IBM PC, and finally entered into Mac domain with OS X 10.1 for home use ... so I fully understand your point.

Apple added a lot of "lock down" processes with, especially starting with macOS Sierra and appears to continue to do so with each new OS addition. I wouldn't be surprised to see even more "mysterious" processes showing up in the Activity Monitor going forward.

I would like to do this... for you to be able to take a look around my mac, but am concerned too about the download aspect. In your first steps above it says download, forgive me if I'm not well versed here, but are you able to do this then w/o downloading? Really appreciate the help...!

First, I suggest that you start a new post and describe the issue or issues that you are having with your Mac. The reason is that the potential solutions to resolve it/them my be different than that of the OP of this post.

FWIW. Installing EtreCheck does not place any hidden files on your Mac. It can easily be completely deleted by just dragging it to the Trash. What is really helpful is that it creates an in-depth report on the health of your computer and provides a report, which you can elect (or not to) share with the community for us to help you.

...does not appear on my iMac which is still running High Sierra, so it's obviously something new related to Mojave on my MacBook...

Good computing Steve Ascheman !