PRIMARY KEY VS UNIQUE KEY

What are the differences between PRIMARY KEY and UNIQUE KEY [PRIMARY KEY VS UNIQUE KEY]?

	PRIMARY KEY	UNIQUE KEY
Use	PRIMARY KEY is used to identify a unique row (record) in a table.	UNIQUE KEY is used to prevent duplicate values in a column.
INDEX	By default, SQL-Engine creates CLUSTERED INDEX on PRIMARY KEY column if not exists.	By default, SQL-Engine creates NON-CLUSTERED INDEX on UNIQUE KEY column.
NULL?	PRIMARY KEY does not allow or include NULL value.	UNIQUE KEY allows NULL value at one time just like any other value.
Existence	A table can have at most one PRIMARY KEY.	A table can have multiple UNIQUE KEYs.
Reference	PRIMARY KEY can be made a FOREIGN KEY into another table.	UNIQUE KEY can be made FOREIGN KEY into another table.
Delete	Table or Table row can’t be deleted if PRIMARY KEY made a FOREIGN KEY into another table and if primary key column value is associated between tables.	Table or Table row can be deleted if UNIQUE KEY made a FOREIGN KEY into another table if unique key column value is associated between tables.
Examples	CREATE TABLE [dbo].[Employee_Unique](        [EmployeeID] [int] IDENTITY(1,1) NOT NULL,        [DepartmentID] [int] NOT NULL,        [EmployeeName] [varchar](50) NOT NULL,        [Gender] [char](1) NOT NULL,        [BirthDate] [date] NOT NULL,        [Salary] [decimal](10, 2) NULL,        [Detail] [varchar](max) NULL,        [IsActive] [bit] NOT NULL,        CONSTRAINT UC_Employee UNIQUE (EmployeeID)  ) CREATE TABLE [dbo].[DegreeEmployee_Unique](        [DegreeID] [int] NOT NULL,        [EmployeeID] [int] NOT NULL,        [Sequence] [tinyint] NOT NULL,  CONSTRAINT [PK_DegreeEmployee_DegreeID_EmployeeID] PRIMARY KEY CLUSTERED (        [DegreeID] ASC,        [EmployeeID] ASC )WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY] ) ON [PRIMARY] GO ALTER TABLE [dbo].[DegreeEmployee_Unique]  WITH CHECK ADD  CONSTRAINT [FK_DegreeEmployee_Unique_EmployeeID] FOREIGN KEY([EmployeeID]) REFERENCES [dbo].[Employee] ([EmployeeID]) GO

[ALSO READ A FEW MORE...]

• PRIMARY KEY vs UNIQUE KEY
• @@IDENTITY vs SCOPE_IDENTITY()
• SEQUENCE vs IDENTITY
• TINYINT vs SMALLINT
• SMALLINT vs INT
• INT vs BIGINT
• DECIMAL/NUMERIC vs FLOAT
• FLOAT vs REAL
• DECIMAL/NUMERIC vs REAL
• SMALLMONEY vs MONEY
• SMALLDATETIME vs DATETIME
• DATETIME vs DATETIME2(N)
• DATETIMEOFFSET vs DATE
• DATETIMEOFFSET vs TIME
• SYSUTCDATETIME vs SYSUTCDATE
• CHAR vs NCHAR
• CHAR or VARCHAR
• VARCHAR vs NVARCHAR
• VARCHAR vs VARCHAR(MAX)
• TEXT vs VARCHAR(MAX)
• BINARY vs VARBINARY
• IMAGE vs VARBINARY(MAX)
• CAST vs CONVERT
• LEN() vs DATALENGTH()
• UNION vs UNION ALL
• DELETE vs TRUNCATE
• Temporary Table vs Temporary Variable
• LEFT JOIN vs RIGHT JOIN
• LEFT OUTER JOIN vs RIGHT OUTER JOIN
• RAISERROR vs THROW
• Function vs Stored Procedure
• QUOTED_IDENTIFIER ON vs QUOTED_IDENTIFIER OFF