Constraints

• Primary key:
  

  A Primary key constraint is a column or a set of columns that uniquely identify a row in the table.
  

• Unique Key:
  

  A Unique constraint requires that each entry in a particular column be unique. Each table can have 249 unique constraints.
  

• Foreign Key:
  

  A Foreign key constraint consists of a column or a set of columns that participates in a relationship with a primary key table.
  

• Default:
  

  A Default constraint is a value the SQL Server automatically places in a particular field in a table. A Default value can be a constant, Null or a function.
  

• Check:
  

  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.
  

• Not Null:
  

  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.
  

• Rule:
  

  A Rule constraint ensures that you can apply rules to multiple tables, where as a Check constraint applies only to the table for which you enter them.