A Primary key is a candidate key to uniquely identify each row
in a table.
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.
Unique Constraints
A unique constraint is a key which uniquely identifies all
possible rows that exist in a table.
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
