Session11 - SQL Server - Primary Key

In SQL Server, a primary key is a column or a set of columns that uniquely identifies each row in a table. It provides a way to enforce data integrity and ensure that each record in the table is uniquely identifiable.

Here are some key points to understand about primary keys in SQL Server:

1. Uniqueness: A primary key must contain unique values for each row in the table. No two rows can have the same primary key value.

2. Non-Nullability: A primary key column cannot have null values. It enforces that every row in the table must have a value in the primary key column(s).

3. Single or Composite: A primary key can consist of a single column or a combination of multiple columns. In the case of a composite primary key, the combination of values from all the columns must be unique.

4. Indexing: SQL Server automatically creates an index on the primary key column(s) to enhance data retrieval performance. This index helps in quickly locating specific rows based on the primary key value.

5. Relationship Establishment: Primary keys are often used to establish relationships between tables in a database. By referencing the primary key of one table as a foreign key in another table, you can create connections and enforce referential integrity.

To define a primary key in SQL Server, you can use the "PRIMARY KEY" constraint when creating or altering a table. Here's an example of creating a table with a primary key:


CREATE TABLE MyTable
(
ID INT PRIMARY KEY,
Name VARCHAR(50),
Age INT
);

In this example, the "ID" column is defined as the primary key for the "MyTable" table.

Remember, a primary key is a critical component in maintaining data integrity and ensuring the uniqueness of each record in a table. It helps with efficient data retrieval, establishes relationships between tables, and plays a crucial role in database design and normalization.