Like Article. Last Updated : 11 Oct, Next Generate a combination of minimum coins that sums to a given value. Recommended Articles. Article Contributed By :. Easy Normal Medium Hard Expert. Writing code in comment? Please use ide. Load Comments. What's New. A Foreign Key is a database key that is used to link two tables together. To facilitate keeping key variable names consistent, and to provide for programs that do natural joins, foreign keys have the same name as is used in the table in which they are the primary key.
A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns fields. A primary key, also called a primary keyword, is a key in a relational database that is unique for each record. It is a unique identifier, such as a driver license number, telephone number including area code , or vehicle identification number VIN.
A relational database must always have one and only one primary key. Arguably, the most important characteristic of an entity is its primary key a single attribute or some combination of attributes , which uniquely identifies each entity instance.
Unlike primary keys, foreign keys can contain duplicate values. Though not automatically created for foreign keys, it is a good idea to define them. You can define several foreign key within a table. A primary key is a column or a set of columns in a table whose values uniquely identify a row in the table. A relational database is designed to enforce the uniqueness of primary keys by allowing only one row with a given primary key value in a table.
Primary keys must contain unique values. Neither the PK nor any part of it can contain null values. This is because null values for the primary key mean we cannot identify some rows. Referential integrity requires that a foreign key must have a matching primary key or it must be null. This constraint is specified between two tables parent and child ; it maintains the correspondence between rows in these tables.
It means the reference from a row in one table to another table must be valid. To ensure that there are no orphan records, we need to enforce referential integrity. An orphan record is one whose foreign key FK value is not found in the corresponding entity — the entity where the PK is located.
Recall that a typical join is between a PK and FK. Most relational databases have declarative referential integrity. In other words, when the tables are created the referential integrity constraints are set up. The referential integrity constraint states that CrsCode in the Class table must match a valid CrsCode in the Course table. When setting up referential integrity it is important that the PK and FK have the same data types and come from the same domain, otherwise the relational database management system RDBMS will not allow the join.
Relational database systems are easier to use and understand than other database systems. See Figure 9. Additional foreign key rules may be added when setting referential integrity, such as what to do with the child rows in the Orders table when the record with the PK, part of the parent Customer , is deleted or changed updated. If these are not selected, the system will prevent the deletion or update of PK values in the parent table Customer table if a child record exists.
The child record is any record with a matching PK. In some databases, an additional option exists when selecting the Delete option called Set to Null.
Though this creates an orphan row, it is acceptable. Enterprise constraints — sometimes referred to as semantic constraints — are additional rules specified by users or database administrators and can be based on multiple tables. Business rules are obtained from users when gathering requirements. The requirements-gathering process is very important, and its results should be verified by the user before the database design is built.
If the business rules are incorrect, the design will be incorrect, and ultimately the application built will not function as expected by the users. Business rules are used to determine cardinality and connectivity.
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