A constraint is a property assigned to a column or the set 
of columns in a table that prevents certain types of 
inconsistent data values from being placed in the column
(s). Constraints are used to enforce the data integrity. 
This ensures the accuracy and reliability of the data in 
the database. The following categories of the data 
integrity exist:


Entity Integrity 
Domain Integrity 
Referential integrity 
User-Defined Integrity

Entity Integrity ensures that there are no duplicate rows 
in a table.
Domain Integrity enforces valid entries for a given column 
by restricting the type, the format, or the range of 
possible values.
Referential integrity ensures that rows cannot be deleted, 
which are used by other records (for example, corresponding 
data values between tables will be vital).
User-Defined Integrity enforces some specific business 
rules that do not fall into entity, domain, or referential 
integrity categories.

Each of these categories of the data integrity can be 
enforced by the appropriate constraints. Microsoft SQL 
Server supports the following constraints:


PRIMARY KEY 
UNIQUE 
FOREIGN KEY 
CHECK 
NOT NULL

A PRIMARY KEY constraint is a unique identifier for a row 
within a database table. Every table should have a primary 
key constraint to uniquely identify each row and only one 
primary key constraint can be created for each table. The 
primary key constraints are used to enforce entity 
integrity.

A UNIQUE constraint enforces the uniqueness of the values 
in a set of columns, so no duplicate values are entered. 
The unique key constraints are used to enforce entity 
integrity as the primary key constraints.

A FOREIGN KEY constraint prevents any actions that would 
destroy link between tables with the corresponding data 
values. A foreign key in one table points to a primary key 
in another table. Foreign keys prevent actions that would 
leave rows with foreign key values when there are no 
primary keys with that value. The foreign key constraints 
are used to enforce referential integrity.

A CHECK constraint is used to limit the values that can be 
placed in a column. The check constraints are used to 
enforce domain integrity.

A NOT NULL constraint enforces that the column will not 
accept null values. The not null constraints are used to 
enforce domain integrity, as the check constraints.