You define foreign keys in a database to model relationships in the real world. Relationships between real-world entities can be quite complex, involving numerous entities each having multiple relationships with each other. For example, a family has multiple relationships between multiple people—all at the same time. In a relational database such as Microsoft Access, however, you consider only relationships between pairs of tables. These tables can be related in one of three different ways: one-to-one, one-to-many or many-to-many.
Figure 3. The tables tblPatient and tblConfidential are related in a one-to-one relationship. The primary key of both tables is PatientId.Tables that are related in a one-to-one relationship should always have the same primary key, which will serve as the join column.
Figure 4. There can be many detail lines for each order in the pizza delivery business, so tblOrder and tblOrderDetails are related in a one-to-many relationship.One-to-many relationships are also used to link base tables to information stored in lookup tables. For example, tblPatient might have a short one-letter DischargeDiagnosis code, which can be linked to a lookup table, tlkpDiagCode, to get more complete Diagnosis descriptions (stored in DiagnosisName). In this case, tlkpDiagCode is related to tblPatient in a one-to-many relationship (i.e., one row in the lookup table can be used in zero or more rows in the patient table).
Figure 5. A linking table, tblPtInsurancePgm, is used to model the many-to-many relationship between tblPatient and tblInsurer.In Microsoft Access, you specify relationships using the Edit—Relationships command. In addition, you can create ad-hoc relationships at any point, using queries.
One-to-One RelationshipsTwo tables are related in a one-to-one (1—1) relationship if, for every row in the first table, there is at most one row in the second table. True one-to-one relationships seldom occur in the real world. This type of relationship is often created to get around some limitation of the database management software rather than to model a real-world situation. In Microsoft Access, one-to-one relationships may be necessary in a database when you have to split a table into two or more tables because of security or performance concerns or because of the limit of 255 columns per table. For example, you might keep most patient information in tblPatient, but put especially sensitive information (e.g., patient name, social security number and address) in tblConfidential (see Figure 3). Access to the information in tblConfidential could be more restricted than for tblPatient. As a second example, perhaps you need to transfer only a portion of a large table to some other application on a regular basis. You can split the table into the transferred and the non-transferred pieces, and join them in a one-to-one relationship.
Figure 3. The tables tblPatient and tblConfidential are related in a one-to-one relationship. The primary key of both tables is PatientId.
One-to-Many RelationshipsTwo tables are related in a one-to-many (1—M) relationship if for every row in the first table, there can be zero, one, or many rows in the second table, but for every row in the second table there is exactly one row in the first table. For example, each order for a pizza delivery business can have multiple items. Therefore, tblOrder is related to tblOrderDetails in a one-to-many relationship (see Figure 4). The one-to-many relationship is also referred to as a parent-child or master-detail relationship. One-to-many relationships are the most commonly modeled relationship.
Figure 4. There can be many detail lines for each order in the pizza delivery business, so tblOrder and tblOrderDetails are related in a one-to-many relationship.
Many-to-Many RelationshipsTwo tables are related in a many-to-many (M—M) relationship when for every row in the first table, there can be many rows in the second table, and for every row in the second table, there can be many rows in the first table. Many-to-many relationships can't be directly modeled in relational database programs, including Microsoft Access. These types of relationships must be broken into multiple one-to-many relationships. For example, a patient may be covered by multiple insurance plans and a given insurance company covers multiple patients. Thus, the tblPatient table in a medical database would be related to the tblInsurer table in a many-to-many relationship. In order to model the relationship between these two tables, you would create a third, linking table, perhaps called tblPtInsurancePgm that would contain a row for each insurance program under which a patient was covered (see Figure 5). Then, the many-to-many relationship between tblPatient and tblInsurer could be broken into two one-to-many relationships (tblPatient would be related to tblPtInsurancePgm and tblInsurer would be related to tblPtInsurancePgm in one-to-many relationships).
Figure 5. A linking table, tblPtInsurancePgm, is used to model the many-to-many relationship between tblPatient and tblInsurer.