There are six types of constraints in SQL Server.

• PRIMARY KEY – It doesn’t allow Null values. A table can have only one PRIMARY KEY.
• FOREIGN KEY – Ensure the referential integrity of the data in one table to match values in another table.
• UNIQUE KEY – Allows Null value. But only one Null value. A table can have more than one UNIQUE Key Column[s].
• NOT NULL – Indicates that a column cannot store NULL value.
• CHECK – Ensures that the value in a column meets a specific condition.
• DEFAULT – Specifies a default value when specified none for this column.