Ms Access Foreign Key
Access is designed as a simple way to build databases for people who are not necessarily technically minded. You do not have to learn an extensive and complicated coding language to create or manage the database. It has an advantage over excel as statistics can be outputted along with reports at the touch of a button via a professional front-end you can create through the use of simple wizards. It is far easier to manage and protect the data in Access than in Excel and can provide a professional front end to your data allowing complete data entry control. There are a few concepts it is important to understand before designing and building the database.
One such concept is the use of Primary and Foreign Keys.Data is stored in access databases in tables. A table is a list of rows and columns containing data. There are usually lots of tables per database. Imagine you needed to build a simple database of customers. The first table you would build is the Customer table, which we will call 'TblCustomer.' You must break down the data to its simplest form and not attempt to put all the information into one large table.
Consider only the simplest information about the customer. In this table each Customer will have its own record, one record only per customer.
Do not be tempted to put all the customer's sales etc in this table as the table will be far too large and impossible to manage. You will also have more than one record per customer and as this table is going to be used to define each customer in the database, that is unacceptable. In TblCustomer will be the first name, surname, first line address, second line address, Town, County, Postcode and an ID number that will be an automated number unique to the customer. This number is called a Primary Key. The most important thing to remember is that this number is always unique to each record.
No two records will ever have the same number as a Primary Key.One advantage of having a Primary Key per record is that this number can be placed in other tables as a 'Foreign Key.' If you were to create a table containing all the sales records, there would be a lot of sales information. It would be impractical to have to repeatedly add customer names and addresses in this second table. It would be a waste of space, disorganised and unnecessarily repetitive. It would also lead to duplicates since spelling mistakes on the name, for example, may lead to the same customer being entered several different times. When a report is created to show the sales per customer, one customer may have several different totals due to such errors.
Ms Access Add Foreign Key
Database cleansing to eradicate such problems is a difficult and expensive process so it is preferable to use the Primary Key and Foreign Key method to link tables as a preventative measure.The Foreign Key is essentially the Primary Key from one table placed in another table in order to join them. When creating a report to see how many sales there are per customer, access will look at a sales record, see that there is a Foreign Key and look in the relating customer table at the matching number to obtain the customer details. Data only needs to be entered once.
When creating a front end for the users to enter the sales, a field can be placed on a form, which will automatically show the customer names, and the user can simply choose from the list. Behind the scenes, access will have placed the Primary Key for the selected customer in the Foreign Key field on the sales table.The Primary Key is therefore vital to define individual records. In reality this is vital when there are likely to be extremely large databases containing thousand if not millions of records. The Foreign Key method allows cross-referencing of tables and records, which is particularly important in modern databases, which can contain thousands of separate tables. The cross-referencing and joining of the tables create relationships between the tables, which is what makes a relational database possible and practical.Author is a freelance copywriter.
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A foreign key (FK) doesn't get a key icon. A FK is not a particular type of field, it's just how the field is being used. If you look at the Relationships Window you can identify PK's by the key icon, and FK's by the fact that they're at the end of an arrowfrom the PK in the related table. You must, however, check the 'Enforce Referential Integrity' button on the Join Type popup when you create the relationship.A multifield primary key is easy to create: just ctrl-click all the fields comprising the key and then click the Key icon on the ribbon. If you are joining a multifield PK to a multifield FK, you must drag all of the fields from the parent table to the correspondingfield in the child table (and click the Enforce button). I am not a fan of composite keys.
They will cause you more headaches then they solve.Use an Autonumber field to generate a PK that can be used as a foreign key. If you need to impose uniqueness on a combination of fields, you can use a multi-Field unique index to do that. This is done in the Index function by selecting multiple fields undera named index with the name blank and seeting No Dups for the named index.
Hope this helps, Scott Blog: Microsoft Access MVP since 2007. To create a composite primary key on two or more columns (fields), in table design view hold down the Ctrl key and left click on each of the columns, being sure to click on the 'field selector' (the small square to the left of the column name). Then rightclick and select 'Primary Key' from the shortcut menu. You should then see the key symbol against each of the columns.However, a table might have more than one candidate key, so alternative keys, whether single column or multi-column, must be defined as such by their inclusion in a unique index. For instance, it might be necessary to create a unique index on the EmployeeID,ProjectID and CityID columns in a ProjectEmployees table to prevent the same employee being assigned to the same project in the same city more than once. In pre 2007 versions this is done in table design view by selecting Indexes from the View menu; in laterversions by selecting Indexes on the Design ribbon.
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In the dialogue which opens enter a suitable name for the index in the first empty row of the Index name column, ProjectEmployee say. In the Field Name column of the same row enter ProjectID, and with thisrow still selected, in the Index Properties below, select 'Yes' as the Unique property. Then enter EmployeeID in the Field Name column in the row immediately beneath ProjectID. Don't enter anything in the Index name column for this row.
Then enter CityIDin the Field Name column in the row immediately beneath EmployeeID. Again, don't enter anything in the Index name column for this row. Close the dialogue and save the table design.BTW the hypothetical example cited above is purely for illustration purposes. Such a table would almost certainly not be normalized to Fifth Normal Form (5NF) and would be in need of 3-decompostion. In the rare situations where such a table is normalizedto 5NF, it would make more sense to designate the three columns as the composite primary key. Adding a surrogate autonumber column as the primary key would serve no purpose, as it is very hard to envisage a situation where the ternary relationship type modelledby such a table would be referenced by another table.
An additional autonumber column would merely add an additional and unnecessary burden, albeit a relatively small one, on the database engine in the maintenance of a further index.While a foreign key is not specifically indicated as such in the visual representation of a table definition in the table design window, the visual representation as such being in the relationships window, it is designated as such in DDL. The following isan example from one of my demo files:CREATE TABLE DrugForms(DrugName VARCHAR(255) NOT NULL,DrugForm VARCHAR(255) NOT NULL,CONSTRAINT PrimaryKey PRIMARY KEY (DrugName, DrugForm),CONSTRAINT DrugsDrugFormsFOREIGN KEY (DrugName) REFERENCES Drugs (DrugName),CONSTRAINT FormsDrugFormsFOREIGN KEY (DrugForm) REFERENCES FormTypes (DrugForm));BTW, drug form in the above refers to the form in which the drug is available, e.g. Tablet, capsule etc. Ken Sheridan, Stafford, England 'Don't write it down until you understand it!' - Richard Feynman.