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.