If you have the SharePoint Enterprise version, you can look at using the Business Data Catalog. This will let you bind columns to external data sources. This might provide you with the functionality you're looking for.
If you do not have the Enterprise features, do you have access to deploy WSP packages and custom code?
You will have to write your own data access to your external data source. Your options would be to have a job that pulls data from the external data source and populates SharePoint list(s) or create a custom view that pulls the external data on-demand.
You'll have to come up with synchronization strategies. Meaning, is the data in the external SQL data source static, reference information that does not need to be updated depending on what a user does in SharePoint? This seems to be the case based on your question. If you do need to update the external data source, you'll have to hook into the on save event (so probably a custom event handler that listens for ItemAdding) to update the data, validate, and optionally cancel the operation with an error message.
If you can't deploy WSP packages / DLLs, you could take a look at the jQuery SharePoint library. This will let you interact with lists using jQuery. If you also write a WCF or Web Service wrapper around the data you need access to from your external data source that is accessible from the SharePoint environment, you can use hack together a solution.
To accomplish this you'd need to place a Content Editor Web Part on the page you need custom data access. In there you will write the code to reference the jQuery javascript library and jQuery SharePoint library. The code will have to make the calls to your external data service and make any updates you need.
This is the least reliable method to accomplish what you want since it's entirely page-based and can be broken by simply disabling script or someone editing the CEWP or removing it altogether.
If you don't have access to place a CEWP or any of the other solutions, then you have no options at all.