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SQL Server constraints

Constraints:

Constraint	Description
Primary key	Uniquely identifies each row. There can be only one Primary key in a table. It cannot have null values. It creates a clustered index by default. It cannot be a foreign key
Foreign key	It is a column or combination of columns that establishes a relationship between 2 tables. It ensures referential integrity which is the accuracy and consistency of data within a relationship. By using cascading referential integrity constraints actions can be defined on a foreign key relationship
Unique	Restricts column to contain unique values
Check	Restrict the values that can be added to a column
Default	Specifies the default values
Not Null	Specifies that the column does not accept null values

Cascading actions:

On Delete Cascade	When rows in the Parent table are deleted then the corresponding rows in the referencing table are deleted
On Update Cascade	When rows in the Parent table are updated then the corresponding rows in the referencing table are updated
On Delete No Action	When rows in the Parent table are deleted then an error is raised and the delete action is rolled back
On Update No Action	When rows in the Parent table are updated then an error is raised and the update action is rolled back
On Delete Set Null	When rows in the Parent table are deleted then the corresponding rows in the referencing table are set to Null
On Update Set Null	When rows in the Parent table are updated then the corresponding rows in the referencing table are set to Null
On Delete Set Default	When rows in the Parent table are deleted then the corresponding rows in the referencing table are set to Default
On Update Set Default	When rows in the Parent table are updated then the corresponding rows in the referencing table are set to Default