Access Help

We need to step back and establish a better understanding of how a relational database application, like those created with Access, need to be designed and built. The way this question is framed suggests that you've not yet mastered that practice. 

All relational databases are based on data stored related tables. You have two to start with; there will be more, no doubt. 

The relationship between those tables is that the Things recorded in some (unnamed) table include Components listed in a second table. 

I.e. "Each TableOneThing includes one Component". 

Or "One Component is included in one or more TableOneThings."

The relationship is "includes". The relationship type is "One to Many".

To understand this better, I recommend you invest time in learning about Database Normalization. That's the method used to create properly structured relational tables. Search for Database Normalization--there are other ways normalization is used.

As Kidd_Ip stated, you need to designate the relationship formally. You do that in the relationship window.

Here's a screenshot showing the relationship between two tables. Note that Referential Integrity is enforced.

That means only values already in the CompanyTypes table are allowed in the CompanyTypeID field in Companies. Access will raise an error and refuse to save any other value in that field.

CompanyTypeID is the Primary Key of the table called CompanyTypes. It's an AutoNumber. I think your Component# is not an AutoNumber, based on the description. I think it probably means some number assigned by your organization for each standard component you use. It'll work as long as every record in the Component table has a unique value for Component#. But it must be designated as the Primary Key, or PK.

CompanyTypes is the "one-side" table I mentioned above, corresponding to your Component table.

One or more Companies can have the same value for CompanyTypeID. That's a Foreign Key, or FK, in the Companies table. Companies is the "many-side" table in this relationship.

In your case, the TableOnethings is the "many-side" table in the relationship with Components. Your FK is again Component#. 

I strongly recommend you change the name from Component# to something less likely to cause problems, such as ComponentNbr. The reason for that is Access doesn't like most non-Alpha characters in object names and having something like the octothorpe, #, as part of a name can result in subtle errors. Avoid that.

The rest of Kidd_lp's comments are appropriate for this task as long as our assumptions about the tables are valid. That is to say, if our assumption that you have a one-to-many relationship is correct, then the combobox on a form will be appropriate. 

Finally, all data entry needs to be done through forms, never directly in tables. 

Forms allow you to put structure around the data entry so that you can do things like offer a combobox of valid ComponentNbrs, use code to manage the process and control the flow of steps and so on.