Keys and Relationships
In SQL Server 2008, keys and relationships play an important role in database design and implementation.
Keys are used to uniquely identify rows in a table and enforce data integrity by ensuring that each row has a unique identifier. There are several types of keys in SQL Server, including primary keys, unique keys, and foreign keys.
A primary key is a column or set of columns that uniquely identifies each row in a table. It can consist of a single column or a combination of columns, but it must be unique and not null.
A unique key is similar to a primary key but can allow null values. It also enforces uniqueness but does not necessarily serve as the primary identifier for the table.
A foreign key is a column or set of columns that refers to a primary key or unique key in another table. It establishes a relationship between two tables and ensures data consistency by requiring that values in the foreign key column(s) match values in the primary key or unique key column(s) of the referenced table.
Relationships in SQL Server refer to the associations between tables that are based on their keys. There are several types of relationships, including one-to-one, one-to-many, and many-to-many.
In a one-to-one relationship, each row in one table is related to one row in another table. In a one-to-many relationship, each row in one table can be related to multiple rows in another table. In a many-to-many relationship, multiple rows in one table can be related to multiple rows in another table.
In summary, keys and relationships are essential components of a well-designed SQL Server database. They help ensure data integrity, enable efficient querying, and facilitate the creation of meaningful associations between data.
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