SQL

Primary Key:-

• A column or a group of columns in a table which uniquely identifies a row. 
• It can not accept null value
• It is created as
  CONSTRAINT pk_name PRIMARY KEY (column1, column2, ...)
• A table can have only one primary key column(s)
• It created a clustered index by default
• It can be referred as a foreign key in some other table

Unique Key:-

•  A column or a group of columns in a table which uniquely identifies a row.
•  It can  accept null value
• It is created as
  CONSTRAINT uk_name UNIQUE (column1, column2, ...)
• A table can have more than one unique key column(s)
• It creates a non clustered index by default
• It can not be referred as a foreign key in other table