Primary Key and Foreign Key in DBMS
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Primary Key and Foreign Key in DBMS
In a database management system (DBMS), a primary key is a unique identifier for a table or relation, while a foreign key is a reference to a primary key in another table.
A primary key is a column or a combination of columns that uniquely identifies each record in a table. The primary key must be unique and not null, meaning it cannot have any duplicates or missing values. The primary key can be a single column or a combination of columns that together uniquely identify each record in the table. It is used to enforce data integrity by preventing duplicate records and ensuring that each record can be identified uniquely.
A foreign key is a column or a set of columns that refer to the primary key of another table. A foreign key establishes a link between two tables, allowing data to be shared between them. The foreign key column in the referencing table must contain values that match the primary key column in the referenced table. This creates a relationship between the two tables, and ensures that data in the referencing table is consistent with the data in the referenced table.
Foreign keys are used to enforce referential integrity, which means that the data in the referencing table must always match the data in the referenced table. If a record is deleted from the referenced table, any records in the referencing table that refer to that record will be deleted as well (this is known as cascading deletion). This ensures that the database remains consistent and prevents data from becoming orphaned or inconsistent.
It is important to note that foreign keys can only refer to a primary key or a unique key in another table. This ensures that the referenced data is unique and can be identified uniquely. If there is no unique identifier in the referenced table, it may be necessary to add one in order to create a foreign key relationship.
In summary, primary keys and foreign keys are essential concepts in DBMS that help to ensure data integrity and consistency. A primary key is a unique identifier for a table or relation, while a foreign key is a reference to a primary key in another table. Primary keys ensure that each record in a table can be identified uniquely, while foreign keys establish relationships between tables and ensure that data is consistent across tables.
Primary Key and Foreign Key in DBMS
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