A Primary Key is a candidate key to uniquely identify each row
in a table. A table is allowed one Primary Key constraint, and a
column that is part of the Primary Key constraint cannot take null
values. Because Primary Key constraints ensure unique data, they
are most often defined on an identity column. To create a Primary
Key Constraint, enter a Primary Key Name in the name column of the
Constraints table, then select Primary Key from the Type Column
dropdown. In the Constraint Definition pane, select the table
columns from the table that are part of the Primary key. A Primary
Key Constraint is represented by a table icon with gold node when
viewed in the Constraints container in the Schema Browser.
Foreign Key Constraints
A Foreign Key is a referential constraint between two tables.
The foreign key identifies a column or a set of columns in one
(referencing) table that refers to a column or set of columns in
another (referenced) table. The columns in the referencing table
must be the primary key or other candidate key in the referenced
table. The values in one row of the referencing columns must occur
in a single row in the referenced table. To create a Foreign Key
Constraint, enter a Foreign Key Name in the name column of the
Constraints table, then select Foreign Key from the Type Column
dropdown. Select the Reference Schema and Reference Table. In the
Constraint Definition pane, select the columns from the tables that
are set in the Reference Table of the Foreign Key. A Foreign Key
Constraint is represented by a table icon with gold node and black
square when viewed in the Constraints container in the Schema
Browser.
Unique Constraints
A Unique Constraint uniquely identifies all possible rows that
exist in a table. Unique Constraints make sure that no identically
copied values are entered in specific columns that do not
participate in a primary key. Multiple Unique Constraints can exist
on a single table. To create a Unique Constraint, enter a Unique
Constraint name in the name column of the Constraints table, then
select Unique Constraint from the Type Column dropdown. In the
Constraint Definition pane, select the columns from the columns
needed. A Unique Key Constraint is represented by a table icon with
crossed node when viewed in the Constraints container in the Schema
Browser.
Check Constraints
A Check Constraint (also known as table check constraint) is a
condition that defines valid data when adding or updating an entry
in a table of a relational database. Within the Constraints Tab of
Create or Alter Table, the Check Condition column is where the
condition is set, determining what the table considers valid data
for the selected column. To create a Check Constraint, enter a
Check Constraint Name in the name column of the Constraints table,
then select Check Constraint from the Type Column dropdown. Enter
the formula for the check in the Check Condition field. A Check
Constraint is represented by a table icon with gold node and green
check when viewed in the Constraints container in the Schema
Browser.
Sample Table with Constraint Types
The screenshot labeled "Table with constraint types" shows the
Alter Table dialog for a table from the AdventureWorks database in
Microsoft SQL Server 2008.
