What is Primary Key Constraint and Unique Key Constraints?

In SQL, both Primary Key and Unique Key constraints are used to ensure data uniqueness within a table, but they serve different purposes and have distinct characteristics.

Primary Key Constraint

A primary key is a column or a set of columns that uniquely identifies each row in a table. This constraint has several important characteristics:

1. Uniqueness: No two rows can have the same primary key value, ensuring that each record can be uniquely identified.
2. Non-nullability: Primary key columns must be defined with NOT NULL. This ensures that there are no rows without a primary key value, which is essential for the integrity of the identification system within the table.
3. Indexing: By default, a primary key creates a unique clustered index on the column or columns that are defined as the primary key. This not only enforces uniqueness and non-nullability but also improves performance for queries that use the primary key column(s) for filtering.
4. Limitation: A table can have only one primary key constraint. This constraint can consist of single or multiple columns (composite key).

Examples of SQL syntax for creating a primary key are as follows:

-- Creating a primary key on a single column during table creation
CREATE TABLE Persons (
    ID int NOT NULL,
    LastName varchar(255) NOT NULL,
    FirstName varchar(255),
    Age int,
    PRIMARY KEY (ID)
);

-- Creating a composite primary key during table creation
CREATE TABLE Orders (
    OrderID int NOT NULL,
    ProductID int NOT NULL,
    OrderDate date NOT NULL,
    PRIMARY KEY (OrderID, ProductID)
);

Unique Key Constraint

A unique key constraint also ensures that all values in a column or a set of columns are unique across the database table. However, it differs from the primary key constraint in the following ways:

1. Null Values: Unlike primary keys, columns under a unique constraint can accept null values. However, only one null value is allowed if the column is defined as unique.
2. Non-Clustered Index: By default, a unique constraint creates a non-clustered index on the columns, unlike the clustered index of...