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Understand how keys work. Click the Database Tools tab. Drag the field you want to use as a foreign key.
Each table will have one primary key that is unique for each entry. By default, Access creates an ID column that increases in number for each entry. This is set as the primary key. Tables can also have foreign keys. These are fields that are linked with another table in the database. The linked fields would contain the same data.  For example, in your Orders table, you may have a Customer ID field to track which customer ordered which product. You can create a relationship for that field with the ID field in your Customer table. Using relationships helps keep your data consistent, efficient, and readable. Click the Relationships button in the Relationships section. This will open a new window with an overview of all of the tables in the database. Each field will be listed underneath its table’s name. You will need to have created the field for the foreign key before you create the relationship. For example, if you want to use the Customer ID on the Orders table, create a field in the Orders table called Customer and leave it blank. Make sure it is the same format as the field you are linking (numbers in this case). Drop it to the field that you created for the foreign key. Click Create in the window that appears to set the relationship for the fields. A line will appear between the two tables, connecting the fields. Check the box to “Enforce Referential Integrity” when creating the relationship. This means that if data is changed in one field, the other field is automatically updated. This will help keep your data accurate.